Yep, the correct amount is the fee that the market dictates. That is, when other app stores and other payment processes are available and developers can be open

Yep, the correct amount is the fee that the market dictates. That is, when other app stores and other payment processes are available and developers can be open about the fees on each purchase (eg, "Visa/Mastercard +3%, Apple payment +30%") then the market will soon determine what a fair fee is.