This raises the question ‘harder than what’?

The services provided by the App Store would have to be present for the business exist at all.

So without the 30% Apple charges those businesses would certainly not survive unless they paid someone else for the services.

Understood.

But do you know how many businesses would die to be able to sell products that had a 70% gross profit margin?

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.