15% seems reasonable. They're not only charging to cover payment processing, there are salaries to pay for those developing the app stores, the human app reviewers (virtually non-existent in case of Google Play), storage, bandwidth, etc.
Granted, both Apple and Google also earn money from ads (shame on Apple's part). In that case I can sort of see the justification to lower their cut to below 15%.
Even the normal 3% overhead for credit card payment flow charged without these monopolistic practices isn’t reasonable. Even 0.1% allows the bloodsucking rentseekers a gigantic margin, as their marginal costs are approximately ZERO.
15% is pure “you have no other options” robbery, expanding on the same thing pioneered by Visa/MC/AmEx.
All of the expenses you listed are a) trivial and b) already paid for by iPhone purchasers. Apple is a hardware company with the highest margins in the industry.
They’re double dipping, and gouging whilst they do so.
The storage and bandwidth costs of the App Store are the size of a rounding error at this scale. Furthermore, the App Store is a marketing tool and value add for the iPhone, and benefits them directly. To expect app developers to pay for it is insane.