Earlier quoted context omitted.
But do you have to work on the program for forty hours a week or four?
How much do you think is my fair price? I will leave it here because it's weird to higgle on it. If I'm going to be "on call" I expect to get paid and I don't think this is greedy or unreasonable.
You can calculate your expected baseline yearly sales from this, based on how many hours you expect to work bugfixing and improving things on average per year.
You can timebox the work- having a half-dozen services like this is exactly how Panic Inc. came to mass prominence.
Everything depends on the actual amount of time you have to spend on the app, nobody is asking for minimum wage work, that's totally not fair; but perhaps expecting this single project to support you full time is unlikely.
The upshot is that if you're more successful than your baseline, that's "profit", and it's unbounded.