Where did you get 29% from? I’m assuming that you are the only developer working on this (you didn’t say otherwise), for every $1 that someone spends, you get .70 cents.
But then why do you assume i've got no expenses ? Just because i'm the only one working on it doesn't mean i don't have to pay myself something to live. That's actually my point : the app doesn't generate enough revenue to cover my personnal expenses.