Our philosophy is simple — when Apple brings a new subscriber to the app, Apple earns a 30 percent share; when the publisher brings an existing or new subscriber to the app, the publisher keeps 100 percent and Apple earns nothing
[…]publishers may no longer provide links in their apps (to a web site, for example) which allow the customer to purchase content or subscriptions outside of the app.
Publishers must provide their own authentication process inside the app for subscribers that have signed up outside of the app.
So, if I buy content using my Apple ID, the content provider will deduct 30% to Apple; but if I buy the same thing, from the same app, using my Provider ID, the provider will not share the revenue with Apple. It really depends on who’s doing the billing: Apple’s take for billing services is 30%.
I just wonder whether each book bought through in-app purchase is also considered a subscription …