If you think an $x one-time payment is a great reward for making y, then make y and charge $x. Seems like a free opportunity to me. If it's really so simple, you can easily undercut those doing the subscription model.

Unless, of course, $x is not actually that motivating a fee for the work. In which case, it's a bit entitled to expect others to work for a fee that doesn't spur you into action either.