The $75 is an arbitrary number and isn't intended to ever be bought for that... There just is no mechanism currently in our digital goods marketplace to disallow selling to non-premium members. It's intended just as a benefit for premium members.
Premium membership is $25 for a single month, can be canceled right away and also gives you access to the other addons I made that are premium member benefits (for example the master/slave database setup). There is no end date on anything, and what you get during your premium membership month can be used indefinitely.
If a year from now (or whenever) there's a new version that you want because it has some awesome feature, you *would* need to be a premium member to get the update. But either way, the $25 cost for a single month is an indefinite license for as many sites as you own.