The published baseline: Apple's standard commission is 30% of digital-goods revenue, reduced to 15% under the Small Business Program for developers earning under $1M a year — which is nearly everyone reading this. Google Play mirrors the shape: 15% on the first $1M each year. For a small app, plan on 15% plus payment processing being handled for you.
Two boundary rules matter more than the rate. First: digital goods and services consumed in the app must, as the baseline rule, use the store's own billing — you cannot simply drop a Stripe checkout into an iOS app to sell premium features. Physical goods and many real-world services are exempt, which is why shopping apps take cards directly. Second: the boundary itself — external purchase links, "steering" users to your website, web checkout for app content — has been redrawn repeatedly in recent years by courts and regulation, and it differs by region. We are deliberately not printing today's specifics, because they have changed several times and will again; before building a web-checkout flow, read Apple's current rules for your storefronts, not a blog post's memory of them.
The practical takeaway is calmer than the discourse: for a small subscription app, store billing at 15% with the renewal machinery, tax handling and refunds done for you is a reasonable deal, and the infrastructure tools below all work on top of it.