Buy an iPhone for %15 off price if you use Apple's App store or buy it at full price if you don't use Apple's App store.

As in, let Apple subsidize the cost of the phone with the app store, I suspect a significant number of people would go for it even with just a five percent price drop.

The problem currently isn't that Apple is charging what they do for the App store, the problem is they are in effect selling customers of their hardware for that amount.