A typical credit is getting paid in, transacted once, and cashed out. And a transaction with a user ID, destination ID, and timestamp only needs 16 bytes to store. So if you want to track every hundredth of a penny individually, then processing a million dollars generates 0.16 terabytes of data. You want to keep that around for five years? Okay, that's around $100 in cost. If you're taking a 1% fee then the storage cost is 1% of your fee.
If your credits are worth 1/20th of a penny, and you store history for 18 months, then that drops the amount of data 17x.
(And any criticisms of these numbers based on database overhead get countered by the fact that you would not store a 10 credit transaction as 10 separate database entries.)