Silicon Alley Insider reports on a Piper Jaffrey research note estimating that Apple gets an $18 monthly fee from AT&T for each iPhone on its network. That works out to $432 per phone over the length of a two-year contract. Even I can do the math on that: Almost $500 million for the roughly 1.1 million iPhones Apple’s already sold that are still on AT&T’s network (excluding those that have been unlocked or bricked.)

No wonder Apple is so intent on not allowing users to unlock their iPhones for use on other networks — at these numbers, those 250,000 “missing” iPhones represent over $100 million in lost revenue over the next two years.