I think, and this is just my thoughts mind you, that when they lowered down to $15, they got a huge influx of new registrations. Then someone said "ok, that was interesting. Let's drop it one more time and see what happens".
My guess is that they will eventually drop it down to 9.95 as a test, but they may have it go back up first to around 17.
Bob isn't stupid. He knows that unless he can draw people in with low prices, he won't get the customers.