Not much, considering that Apple eats up all the cost for providing all the free apps, handles all the advertising for free, payment processing for free, sells gift cards below face value (because no store would sell $50 gift cards if they just had to pass on the $50 to Apple). Apple makes money by selling hardware. The App Store is basically advertisement to entice you to buy the hardware.
Seriously? Let me respond point-by-point.
- Apple charges $99 for an app developer license.
- Advertising is a fixed cost that gets amortized over their huge revenue stream (which they breakout in their 10-K)
- Apple takes 30% off the top for apps. That more than covers the store's cut on the gift cards.
Apple is now doing a couple of billion dollars revenue per quarter in the App store, so the profits may be closing in on $1 billion on an annual basis.