Play is also an anti-competitive monopoly as well. Google was a bit smarter about avoiding potential anti-trust though, because they at least give the illusion of choice with 3rd party app stores.

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.

> Google was a bit smarter about avoiding potential anti-trust though, because they at least give the illusion of choice with 3rd party app stores.

I'd argue the opposite -- one of Epic's allegations is that Google scuttled a deal Epic had reached with OnePlus to ship Epic's own Android app store.

The illusion of choice has nothing to do with what is the default choice.

The choice is important. About 20% of apps on my Android phone come from F-Droid marketplace distributing free software and some of them are not distributed via Play store at all. And then there are Amazon App Store, Galaxy Store etc. (device-manufacturer stores) https://fossbytes.com/10-google-play-store-alternatives/

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%.

A flat monthly fee

That doesn't answer the question. A flat monthly fee could be $1 dollar or $100,000.