Then Simply decide how much you wish to invest!
I use an EXTREMELY simple method whereby your discount is based on the amount you add to your account. The License fee never changes but based on the amount of your payment I will ADD an amount to your deposit to give you extra funds. The net result is that you can buy more Licenses with the same funds from your PayPal.
This means I do not have to deal with discount levels on an individual basis. KISS, or Keep It Simple Stupid is the basis for the way I live. My programs try to reflect this principle.
Simple does not have to mean inferior, it can mean the program is smaller, faster, and with less code logic, "should" be more stable. Nothing is perfect but I do my best to fix anything I am made aware of.