There's a different kind of registration for that I believe, I have a standard developer program license ($99USD/year), there's another "enterprise program" which I think can do 500: http://developer.apple.com/iPhone/program/apply.html
The enterprise program lets you issue a provisioning profile that's not tied to UUIDs. Translation: you can have as many app users as you want.
On the other hand... there's no easy way to charge. The provisioning profile is all that's needed to run the app.
Also, these profiles are intended for "internal distribution" (inside a company). I'm sure Apple will blow up your certificate if you try public distribution.