The number probably came from looking at what brick and mortar stores do. Typically stores "mark up" products by 40% from their distributors. And if you think

The number probably came from looking at what brick and mortar stores do. Typically stores "mark up" products by 40% from their distributors. And if you think about it, that's effectively what Apple does, but in this case, the distributor (aka app developer) has to "mark-up" the price to cover the cost incurred by selling in the app store. In general this practice is pretty common in retail.