Agree with Keith’s comment: One-off payments - between 20-25%, subscriptions - between 5-10%
Depends on what is being purchased. If this is a one off program or game then I would think 15 or 20 percent might be in order. If this is a subscription like Netflix or HBONow that is charged on a regular basis it should be 5 to 10 percent.
I would support 20% at most. Apple makes an amazing ecosystem, but it's also a very locked down ecosystem. If an app developer doesn't want to sell their apps on the app store, they can't distribute their apps. Apple is raking in billions of dollars for this monopoly and I think it's well worth revisiting this issue. At the very least, APple needs to relax their stance on non-app-store subscriptions. Many apps either need to silently charge extra for a cross-platform service if the user signs up on the phone, or take a 30% cut on a subscription that applies to much more than just iOS. Considering the way most of these apps operate, I'm thinking Apple has a policy against advising people to go elsewhere to sign up for a subscription. I've never gotten an e-mail or a notification that says "If you go here, you'll get it more cheaply, or you'll get these benefits for the same price." Surely developers would take advantage of the opportunity if they were allowed to.
The cost of housing and processing the payments is greater than 5% and the FACT that the developer has an incredible marketplace is WELL worth the total of a 25% cut to Apple.... The developer would NEVER sell as much as they duo without the App Store and deep down they KNOW IT!