The premium is actually higher than that. For $100, the developer gets $70. Which means the premium is $30 on a $70 base price, i.e. a ~42% higher price. That's a very steep price for easier payment cancellation.
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.
The customer paid $100. That is by definition the price. All costs are relative to the price. For example, sales tax or transaction fees.
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%.