Registered developer != paying developer. Think about how many iOS developers a company has, they don’t pay $99/yr for each one, the company instead pays that once (usually) for all of them.

That's a good point. But even if 50% of registered AppStore devs didn't pay their fee, 1B is still a huge pile of cash.

Pure WAG, but I’m guessing <10% of registered developers are paying the annual fee.