I wonder what the cost of building and running the app store is? Apart from the engineering costs and costs to run the servers, there are reviewers to be paid,

I wonder what the cost of building and running the app store is? Apart from the engineering costs and costs to run the servers, there are reviewers to be paid, marketing, gift cards to be distributed, various tax and legal problems that are hidden from app developers and the like. And of course there's Apple's profit margin, which as capitalists I don't think many people are denying outright. It's very hard to separate this from the costs of the iPhone, but ofc since all financial details are private anyway all we have is speculation. What do people think would be a fair 'tax'?