The fee should be whatever would the fee would be, if there were multiple app stores allowed on the iPhone. I expect that number would drop to around 10%, for

The fee should be whatever would the fee would be, if there were multiple app stores allowed on the iPhone. I expect that number would drop to around 10%, for "premium" app stores, such as if Steam or the Epic app store were allowed on iPhones. But also, I would expect there to be even lower priced competitors. Payment processors charge 3% of sales, so a competing app store could get pretty close to around 3%, if they wanted. So the answer is 10% for "premium" app stores, and for the app stores that instead only doing payment processing, the price should be 3%.