I'm not sure trying to figure out the "correct" percentage to cover Apple's costs really makes sense. All those factors you mention are probably better measure

I'm not sure trying to figure out the "correct" percentage to cover Apple's costs really makes sense. All those factors you mention are probably better measured as a flat fee per user rather than a percentage of the sale cost. Free apps cost Apple just as much to host and support as paid apps, so why don't they get charged? The 30% is essentially arbitrary. They're making a healthy profit from the App Store, so 30% must be on the high side. And the amount different apps are paying must vary a huge amount, only loosely correlated with their hosting costs.