Like a load of things on Route 66 the car hire costs come down to how you want to do the trip. For most people doing it in a convertible and probably a Ford Mustang convertible at that is the only way. We did it like that and I don't regret a penny that we spent on it. However you can of course do it in any car that you would like to and then the cost is appropriate!
Of course 99.99999% of people will do the route in one direction (east to west) so you have to be aware of the one way car hire costs. This is always hidden in the "extra's" when you get your price. It's very easy to look at the headline price and think you've got a bargain but then be stung...
We paid about $1450 for the basic hire car and then the extra's for the car hire came in at about $700. The extra's don't really change that much even if you go for a cheaper car. This is is the one way fee, and taxes, airport fee etc.. Be careful about the second driver as well as they were going to charge us for this as well but at the car collection point we managed to get this free of charge.