The fixation on what Apple's costs are in operating the App Store is weird. It reminds me of the people that look at the estimates of the bill of materials for an iPhone and who use that to say Apple's charging too much. That's irrelevant. It's not meant to be run at-cost for the benefit of developers. The App Store is a business to Apple just like any of their other products. The cut that "makes sense" is whatever the market will bear.