If apple made it easy to install other stores. E.g. when you open the iPhone for the first time it would not prioritize the app store (like I think it does with browsers, at least android does).

This would create a more fair market. This would allow for competition. Which would allow prices to even out and be more fair.

Long winded way of saying, I don't know what the fair percentage would be. But I know as it stands now, we cannot even begin to know it.