I think there is also the need to make sure that the equipment only needs to be bought once. I think the minimum you would spend for a touring car that is a real chassis with actual race potential would not be the cheapest possible, but more like a VBC Ghost Evo, which is $200 and would give the buyer a chassis that can win, not just be a starter kit. Also the Hobbypartz gear will get them running but gear just slightly more expensive, like Hobbywing Justock and V10, can get them to the winners circle.
With everything I have seen, and I actually have done this little thought experiment before with various websites, I think $600-$650 would go from a clean table to a car that has everything it needs to win, including some budget tools, tires, body, a select upgrade or two (double cardan joints in the front axles) AND has gear that doesn't need to be replaced to move up in class.