"I have a pretty niche app that's making about $400k/year on iOS"

"I've done almost zero work for the last year"

Really, why sell it then? That seams like a pretty nice buffer from a day job.

Otherwise - try to calculate growth month over month and project the growth a year or 2 into the future minus what your hourly rate might be at a comparable day job to maintain it and ask for the $ you get at the 2 or 3 year out mark? I think I'd be happy with that figure.