I've also thought of an app along these lines. You plug in the overhead to do the gig: gas, tolls, parking, time, etc., the money you have invested in your equipment, the time and effort to put the band together and rehearse, other miscellaneous expenses, and the payment to the band, then hit the button that says How much I'm making and within a few seconds you get the accurate response:
Not enough
hehehehe
Good luck with your app, Jazzy, but, alas, I'm an Android kind of guy.
Bill, tgo