Last quarter, gross margins on "Services" revenue were 75%. The App Store is almost pure profit, with relatively little investment.
See also: https://www.apple.com/app-store/ "Every week, nearly 500 dedicated experts around the world review over 130K apps." Astonishingly, Apple appears to be bragging about these numbers, but if you do the math, 500 reviewers working 40 hours per week doing nothing except reviewing apps—no training, no meetings, no breaks, etc.—must spend an average of less than 10 minutes on each submission, to review the app and the App Store metadata (text, screenshots, etc.) for conformance with all guidelines, not only safety but also, more importantly to Apple, it seems, ensuring that the app doesn't somehow avoid giving Apple its cut of revenue. Not to mention that the salary of 500 reviewers is basically a drop in the bucket compared to App Store revenue.