It's entirely reasonable for apple to get some cut for validating and distributing the app. But a flat 30% (eventually dropping to 15% per customer) doesn't seem right for something charging a significant amount of money.
If the 30% fee applied to the first 10-20 dollars, and they only charged for payment processing after that, they'd still make plenty of money both up front and recurring. (I'm imagining something like "5% flat" or "cost they pay plus 1.5%" for the payment processing fee.)