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.