For brand new cars, FlightCar honestly comes out to about even as you mentioned. Though, our ordinary car is an '08 Prius, which has depreciated enough in value (as cars depreciate quickly!) to make our service a definite positive. Also, our utility will probably never really be 100% (Hertz for example is doing a lot to increase their's and is at 65% or so I believe). So your car most likely won't be rented for everyday you're gone.
We do try and make the experience way better than traditional airport parking. We drop you off and pick you up in a town car and we give you free Starbuck's coffee before you fly and after you land! You also don't come home to an outrageous bill!