Apple has a crazily powerful digital store, used by hundreds of millions of customers. But if you want to sell something there it will cost you: In almost every case, Apple keeps 30 percent of the purchase price on whatever its users buy. Beats Music, the new subscription music service that launched in January, figures that’s worth it. Beats updated its iPhone app today, and now lets users subscribe to the $10 a month service from within the app, using iTunes’ billing system. That means Beats, and the music labels that license their stuff to the service, will give up $3 a month on every subscription sold through iTunes. Beats CEO Ian Rogers says the decision to sell within the Apple app was fairly straightforward: More than half of Beats users use iPhones, and it’s very hard to get an iOS user to subscribe if you don’t sell in-app. Two other music subscription services — Rhapsody and Rdio — have also agreed to sell subscriptions within Apple’s app, though Rdio raised the price for for in-app subscriptions from $10 a month to $15 a month to accommodate Apple’s tariff.