Earlier quoted context omitted.
It is the percentage of the market price that the customer paid, not what developer got. You're twisting this. Think about it this way - if Apple was not in the middle, the developer would have received $100. So it is 30% less than what they would have received without Apple cut.
They meant the user pays a 42% premium. If the developer needs to earn $70 they need to charge $100 on the App Store which is a 42% increase for the user. So if you’re talking about passing along those costs it’s +42% on every purchase.
The customer paid $100. That is by definition the price. All costs are relative to the price. For example, sales tax or transaction fees.