If a developer wants to sell something via an iPad app - it's called an 'in-app purchase' - the transaction must go through Apple, which keeps 30 percent of the money and passes 70 percent on to the developer.
“If a developer wants to sell something via an iPad app - it's called an 'in-app purchase' - the transaction must go through Apple, which keeps 30 percent of the money and passes 70 percent on to the developer.”
— Daniel Lyons · App