Seems to me the range of what a credit card charges would seem sensible. I.e. 3-7%.
Alternatively charge 30% but be forced to allow side-installing Apps.
That way developers can decide if they want the convenience and reach of the AppStore or not.
Sure. I think side loading is the best options or allowing another way to pay where apple takes a lower cut makes sense. I'd argue that credit card companies are doing a lot less than a marketplace? But again... How do you come up with a number that really makes sense? Apple is just going to charge whatever they can until they are told they have to change.
Alternatively charge 30% but be forced to allow side-installing Apps. That way developers can decide if they want the convenience and reach of the AppStore or not.