cup cars cost 63k new. we can definitely build a car for less than that, depending on a number of variables, i think mid 30s is a realistic number for a well put together car.
the cage design on the cup cars is great for close racing in mx5 cup, but really intrusive from an aerodynamic perspective, the hard top for that fits the cage of the cup car is 3k, that is pretty steep compared to 1200 for a non cup car hard top.
i do not want to put the wear and tear on my cup car to run in the cup series after an enduro, you need to consider changing hubs, transmission, motor, rebuild shocks etc.... these costs add up pretty quick. also we wont need to run sealed motors, diff, transmission etc in a WRL/NASA car.
the cup car is put into TTC for nasa, the tire the cup car runs really pushes it over the TTD limit. with the same HP/Weight ratio better aero and running an RR or hoosier we could sit atop the TTD.