Most of Apple's profits come from hardware. In software, you should really go by software revenues. There's an installed base (units), but the installed base is

Most of Apple's profits come from hardware. In software, you should really go by software revenues. There's an installed base (units), but the installed base isn't the market, just the potential market. The market consists only of people spending money... Having said that, you need to account for all the revenues, ie not just the purchase price of an app in the store. That's an area where not everybody is happy with Apple.