A reasonable percentage is the one that Apple would charge if it had competition. Apple obviously cares about user experience, and does not want iOS apps to be split between multiple stores, so if they had to allow alternative install sources they would likely drop their fees by a
lot, to make sure that developers have no incentive to promote alternative app stores. That lowered fee is the one developers should have been paying all along.
If you really put the squeeze to them, the "natural" price would probably be a bit below cost for them, because the availability of apps is a selling point for their high-margin phones and tablets. That is why they are willing to host free apps, after all. (Although even then they take the developer fee)