My car has cost me £0.65/mile for all costs excluding fuel (which would I imagine be the big cost difference between the UK, where I am, and America). I don't believe my car is even terribly expensive as cars go - TCO over 2 years so far would barely buy you a small hatchback - but at $10/day, I'd still be losing out simply based on the valet driver driving it ;)
OK, so maybe if you ignore the cost of the car/insurance/car tax (since I would pay these anyway, whether I hire the car out or not), and focus only on repairs and servicing, because these are roughly proportional to distance driven, the cost is £0.07/mile. 90*0.07 = £6.30, which is pretty much $10.
Perhaps my car is just more expensive to run than average (perhaps due to its age), or I take needlessly good care of it for a second-hand one, or whatever, and/or maintenance is super-cheap in America. But add in the risk of your average rental driver taking less care of your car than normal, quite apart from the bother of sorting out speeding and parking tickets, and it's starting to feel like I wouldn't be earning enough money from it to make up for the potential hassle.