I've got no data on it, but my gut tells me it's a tiny, tiny percentage. I would also bet that switching the entire massive operation over to using pentalobe screws (not to mention the presumably higher cost of the screws themselves) cost more than the revenue they're "losing" to people who would repair old phones themselves.
Isn't it more likely that the pentalobe screws, with higher internal surface area than a phillips screw, offer some other benefit? Looking at them I would bet that they stay on automatic screw drivers better than a phillips-head screw.
If they really wanted to make it impossible to un-screw them they'd make a screw whose head was in the shape of an Apple logo or something that nobody makes a driver for. It's not really hard to find a pentalobe screwdriver so if they were trying to keep people out it's not working. (Actually, using an Apple logo as the screw-head shape would be kind of interesting, because they might be able to sue anyone who made a matching driver for using their trademark...)