A percentage works fine, but you need a competitive market for that percentage.

> And Apple absolutely does increase your reach. Not in any important technical sense, but by creating a low-friction, high-trust environment, which results in a much larger market for apps than would exist if purchasing required users to type their credit card numbers into your web site and download and run an installer.

Plenty of companies will charge 5% or less for low-friction high-trust payment processing, and the auto-install is trivial.