¶ FooPub Inc. publishes FooMag through the App Store, at a price of $1/year.
¶ Each issue of FooMag contains some small amount of content and a link: “To read more, go to foopub.example.com and subscribe to FooMag Deluxe for $20/year!”
¶ Users download FooMag Deluxe through FooPub’s own servers, bypassing the App Store completely.
¶ Scads of people discover FooMag through the App Store, subscribe to it (giving Apple 30¢ out of each $1 subscription), and a significant fraction of them upgrade to the deluxe version (giving Apple nothing).