But they're not billing on the basis of how much it technically costs them to provide these services. If we had a competitive ecosystem then we would expect App

But they're not billing on the basis of how much it technically costs them to provide these services. If we had a competitive ecosystem then we would expect Apple's prices for payment processing to be at least within an order of magnitude of (for example) Stripe's. Of course security screening is expensive, but it's also not that expensive (e.g. a typical software company might have a 10-30% profit margin, so in some cases apple accounts for roughly half the operational expenses of a company – i.e. the company pays as much, or more, money to apple as it does to its entire payroll)