No idea what to upgrade? Not sure what to do with FSP? Well, let me explain each of the upgrades.
The upgrade costs FSP, not gold! If you want to know what FSP is, then it's a premium currency. It allows you to upgrade your character. That's not all! You can use it for Hell Forge and Crafting to make your gears more powerful. You also can donate FSP to guild to help upgrade guild.
Cost: The amount of FSP needed for upgrade.
Limit: How many times you can make an upgrade.
Cost to max upgrade: Only applies to upgrades that have an limit. It show how much FSP is needed to max the upgrade.
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25 current
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Cost:
1
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Limit: Infinity
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I don't recommend you use this. It's expensive. Besides, it take only 1 hour to regenerate much more
stamina than you get with 1 FSP.
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10 maximum 
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Cost: 2 
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Limit: Infinity
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This is a must-have upgrade. It increases the maximum stamina, allowing you to play longer. It also increases the efficiency. If you have a maximum stamina of 2000 and above, it would take more than a day to completely refill.
It typically takes about 10 minutes to burn 500 stamina, if you use the shortcuts key to rapidly kill enemies. Certain items will raise max Stamina.
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1% XP towards next level
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Cost:
2
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Limit: Infinity
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I don't recommend this upgrade either. It only increases 1% XP towards your level. Besides, the 25
stamina is much efficient than 1% XP.
2 FSPs get you 50
stamina. You could typically raise more XP than 2 FSP for 1% XP towards next level.
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50,000 
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Cost: 1 
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Limit: Infinity
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I don't recommend this upgrade. Why not sell FSP at marketplace instead? This upgrade only give you 50,000 gold. Check the marketplace. You can get like 155k gold per FSP.
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25 XP
Gain per Hour
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Cost:
15
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Limit:
50
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Cost to max upgrade:
750
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This is a good upgrade. It permanently raises XP
gain. The actual value is shown on mouse-over. XP
Gain is allocated, plus a bonus from equipment and multiplier, which is level divided by 25 (rounding down).
Below level 25, no additional bonus is applied. At low levels, XP
Gain is reduced by a factor 10 to prevent from being overpowering. And if you max out the upgrade, the XP
gain is 1,252. I am pretty not sure how multiplier works, check the mouse-over on description in the upgrade pages. Or ask HCS.
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10
Gain per Hour
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Cost: 5 
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Limit: 50
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Cost to max upgrade: 250 
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May be a good upgrade. It increases the amount of Gold printed per hour by 10. If you max out the upgrade, 505 gold will be printed per hour. Which means you will be able to make 12,120 gold per day or 84,840 gold per week. Useful if you don't log in often.
However, I don't recommend this upgrade. So I give it 2/5.
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1
Gain per Hour
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Cost:
25
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Limit:
25
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Cost to max upgrade:
625
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Not good unless you purchase in multiplies. It increases the amount of
stamina you will
gain per hour. If you max out this upgrade, you will regain 75
Stamina per hour. If Endurance Shrine is at level 3, it will push the
Stamina Gain to 90. Epic
items usually raise
Stamina Gain. If you equip the
items as explained in this thread, you will regain a whopping 108
Stamina per hour! Better yet, if your guild has 6 relics that boosts
stamina gain and have empower level of 10, you will be able to regain even more whopping 120!
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1 Bank Deposit per Day
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Cost: 50 
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Limit: 10
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Cost to max upgrade: 500 
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Only use this if you have a lot of gold you need to store. If you max out the upgrade, you will be able to deposit 11 times per day.
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1

Slot
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Cost:
15
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Limit:
1,000
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Cost to max upgrade:
15,000 
(This is the most FSP required to max out this upgrade!)
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A very useful upgrade indeed. It lets you carry more
items. You start with 3 slots. The first few upgrades are most useful. I recommend you upgrade 6 times, up to 9 inventory slot. If you max out this upgrade, you will have a whopping 1,003 inventory slots! These upgrades are useful for Epic quests.
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1 Max Allies
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Cost: 5 
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Limit: 115
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Cost to max upgrade: 575 
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Useful if you want to be popular. If you max out the upgrade, you will be able to have up to 120 friends. You start with 5 friends slot.
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1 Max Enemies
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Cost:
5
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Limit:
115
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Cost to max upgrade:
575
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Same reason as 1 max allies upgrade, except that if you have a lot of players you don't like, this is the upgrade for you. Like the friends list, you start with 5 and can be upgraded to 120.
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1 Max Auctions
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Cost: 20 
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Limit: 100
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Cost to max upgrade: 2,000 
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Useful if you want to sell a lot of items and generate more gold. You start with a limit of 2 and can be upgraded to 102.
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Protect XP
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Cost:
25
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Limit: Infinity
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Only use it if you reach a high level and if players attack you often. Sadly, it does not protect you from XP loss by being killed via a bounty.
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Reset Single Skill
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Cost: 15 
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Limit: Infinity
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Only use it if you have made a mistake on a skill and want to reset it.
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Reset All Skill Points
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Cost:
100
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Limit: Infinity
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Only use it if you made a mistake and have allocated to more than 7 skills. You will get all skill points back. There are free skill resets each time a new skill come out and there's no expiration for free reset.
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1 Max Secure Trades
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Cost: 5 
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Limit: 25
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Cost to max upgrade: 125 
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Great if you're very busy with deals for many players. You start with a limit of 2 Secure Trades and can be upgraded to have a maximum of 27 Secure Trades.
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Refill All Bank Deposit
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Cost:
20
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Limit: Infinity
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Instantly refills all of your bank deposits. Saves more FSP if you have upgraded max bank deposits per day. Only use it if you have a lot of gold needed to be stored and have more than 3 deposits per day.
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100 
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Cost: 1 
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Limit: Infinity
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Gives you 100 Bounty Tickets. Required to take on a bounty or create a bounty. If you're attacked by player and if he/she doesn't return gold to you, consider using this upgrade to bounty him/her.
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Player Reset
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Cost:
150 
25

per use.
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Limit: Infinity
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Resets all of your level-up points. The cost will raise each time you reset, although eventually, after you use this a certain amount of times, the FSP cost will stop raising. Unfortunately, there's no cost limit. And yup, the cost increases by 25 FSP each time you use this so use wisely.
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1 Marketplace Listing
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Cost: 10 
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Limit: 5
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Cost to max upgrade: 50
(This is the lowest amount of FSP needed to max a particular upgrade.)
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Particularly useful if you want to put different gold cost but I don't recommend this upgrade.
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1 Combat Sets
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Cost:
5
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Limit:
49
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Cost to max upgrade:
245
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Only useful if you have a lot of sets you want to use, depend on the situations. You start with only 1 combat set, but you can upgrade to have a total of 50 Combat Sets.
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25 Bio Characters
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Cost: 1 
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Limit: 75
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Cost to max upgrade: 75 
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My favourite upgrade of all time! It expands your biography capacity so you can put more information. Great if you want to inform about buff shop. Also great for telling about your misc statistic. You start with 255 characters limit. Can be upgraded to have a maximum of 2,130 characters!
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1 Component Inventory slot
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Cost:
5
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Limit:
490
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Cost to max upgrade:
2,450
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Useful if you want to make quantities of
items you want to make. If you fail to make an item, you can keep retrying. You start with 10 component inventory slots and can be upgraded to have a maximum of 500 component inventory slots.
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1 Combat Move Sets
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Cost: 5 
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Limit: 18
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Cost to max upgrade: 90 
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Only useful if you have a lot of combat moves and not sure which to use. You start with a limit of 2 and can be upgraded to a maximum of 20.
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That's the end of the list of the upgrades.
So what would I recommend? It all depends on what you want. But avoid the 25 Current
Stamina, 50K Gold, and 1% XP towards level up. It only wastes FSP.
Fact: It costs a total of 23,310 FSPs to max out every upgrades that can be maxed out!