If you want to know how much Exp needed for the rewards
If you're interested to know how much Exp you need to get specific reward, for example Samurai level 100 Sword VFX
Its S(n) = 20n^2 +100n
So if you're level 50 on Samurai and want to know how much exp need for level 100, it will be
-> 20*[100^2 - 50^2] + 100*[100-50]
=> 155,000 Exp required