If there were competition for App Stores, we could discover what the correct market price for App Stores is, but Apple doesn't want that.

This is a very good argument. We would also learn what features of an App Store add marketable value, and what features are trivial. I imagine the front end isn't very important, but some kind of build certification/verification is. That requires branding, infrastructure and labor. Maybe its easier than I imagine to verify that apps aren't lying about what they do, but as far as I can tell that could well account for some 5% at cost.

On the other hand you trust your bank, for example, so you follow the link on their website and install the App, and the trust came from their own brand.