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.

No! The supply-demand curve intersects at $100 and not $70. 70/30 split is an internal transaction and cost of doing business. No different than distributor cut if you’re selling physical products.

The customer paid $100. That is by definition the price. All costs are relative to the price. For example, sales tax or transaction fees.

>if Apple was not in the middle, the developer would have received $100.

You are never going to get 100% because of payment processing, license management, and content hosting costs. If not Apple, then somebody else. You can have a payment processor and manage the licensing and hosting yourself. Or, you could use an all-in-one vendor, but you will still be paying 10-20%.