My gut reaction is that race car rental only makes sense with relatively disposable cars. $4-500 a day for a Miata is not a crazy amount of money, for newbie enthusiasts, and it's hard to do
too much damage in/with/to a Miata. Anyone with a reasonable amount of track experience should be able to drive the car at what feels like 9/10 all day and still not be in a dangerous place.
A well-set-up 911, on the other hand, is going to cost more to operate, cost more to repair, and have a potentially much more dangerous character out on the track for a not-so-experienced driver. Remember, these are rear-engine cars with a tendency to get tail happy. Somebody looking for the '911 experience' could rent one of these for the day and get much more than he bargained for.
Out here, you can rent a new-generation Viper for $900, but you've got to pretty much sign over the deed for your house in order to get it out the door, liability-wise. I'd think you'd have to price a daily 911 rental in the $600-700 range, to be reasonable, and it's hard for me to imagine the income from that being worth the risks.
But then, I've got no first-hand experience with this sort of thing. I always drive my own car.
