At present, all in-app purchases require payment of 30% of list
price to Apple. At present, this represents 100% of the profits
granted to us eBook distributors by the publishers. […]
Overall, I would love to use in-app purchases. I would be happy to
let Apple make some money from that. Just not every single penny
that the publishers allow us to keep from the transaction.
The “agency model” deal from most publishers requires them to receive 70 percent of the sale price. So something has to give if Apple is going to require third-party e-book sellers to use their in-app purchasing system.