I'm not sure about your calculation. A business with only 29% gross profit margin wouldn't be able to survive on the app store because they'd have to give 30% to apple.

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.

The marginal profit is the price the good sold for - the marginal cost. The only thing Apple has to do with this is the 30%.

The only fixed cost imposed by Apple is the $100 developer fee and whatever it cost to buy a Mac.