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.)
I wasn't fully envisioning credits only being transacted once before cashout either. I was thinking more along the lines of being able to create something that goes viral, a lot of people use it and you rack up a bunch of credits, and then you can sit on those credits and spend them as you use the internet without ever having to connect to a bank yourself. So people who are contributing more than they are consuming would rack up credits. they could use those credits to enrich their contributions, maybe pay for cloud services, etc.
the credits could form its own mini web economy if it got popular enough. As cool as this would all be if done honestly, I know that if i saw a company telling me to buy web credits to use anywhere on the internet and the websites get to decide how much to charge and they charge it automatically when i visit the website, and if the company i buy the credits from goes out of business then i may not be able to cash out or get my money back, then I likely wouldnt be buying those credits... so idk