Even framing it as a percentage is a category error. If they're providing services, we should be able to pay for the cost of those services.

Beyond that, the app store is worth something in aggregate to app developers, but it's worth _more_ to Apple. Without the app store the iOS ecosystem doesn't exist. Without Apple, developers work on the next best platform. Under that framing, shouldn't Apple be paying developers to not leave the platform?

The fact that neither of those things is happening (total rent is disproportionately larger than the services provided, and the net flow of cash is toward the beneficiary of value rather than away from it) points to a monopoly worth breaking down. Those aren't hard rules, but they're suggestive facts.

Actually answering your question: Payment processors charge up to 5% and do more due diligence and have more overhead than Apple, so if I had to name a percentage on every transaction (a viewpoint I already disagree with), 5% would be a hard upper bound. IME, the value Apple provides from their "services" in that space is usually negative aside from the fact that you have to do business with them, so I think the actual price should be much less.