Fixed amount for topup

I want to be able to fix the minimum amount a user can spend to topup 1 unit of his wallet. For example. User X wants to top up hi wallet with, say 2AD$, I want to be able to charge user X, say 3US$, per 1AD$ that user X wants to topup on his wallet.

I have a payment gateway setup to accept bitcoin and would love to charge users an amount in Bitcoin for each 1 unit of currency they want to fund their tera wallet with.

Please Help