There basically is not a competitive market for either app marketplace right now, so an accurate price can't be determined. Allow 3rd party marketplaces to be installed as easily as any other app and I'm certain prices will come down from where they are today.

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%.