Most credit card processors charge around 3%, so it's rarely an issue. The number I usually throw out that Apple could charge and no one would be mad over is 5%

Most credit card processors charge around 3%, so it's rarely an issue. The number I usually throw out that Apple could charge and no one would be mad over is 5%. It's more than a credit card processor, but not that much, and it would likely help conversion rates enough that app developers would be happy.