There are only two racing games I can recall which used concept cars in races:
Apex (fictional and real?, XBOX)
X-Car: Experimental Racing (fictional, PC)
I do feel in a way that if I sold a concept car, I would like to be able to maybe get money back from all the tuning. I can recall in the PC game (not the crappy PS1 version) "Sports Car GT." For example, a Porsche 911 GT1 costs about 700K Credits in the game. All of the modifications you give the car will increase the car's price. Then again, I think cars have a sale value based on 50% of its current price. For GT games since GT2, cars were mostly sold based on the starting price of the automobile is usually its original price available at 25%. So you can do the math. If a car sold for 1M Cr, then its sale price (no matter what you do with it) is going to be 1/4 or 25% of its original price. So if a car sold for 1M, you can sell it and get 250K for it. Cars cost more if you are trading from another Memory Card. The cost of the car + tuning adds up to a higher price for the car. Perhaps the car itself can sell for nothing, but the tuning would probably or should sell for half the total cost of tuning.
Maybe a reason that Concept Cars cannot be sold for any money is because many concept cars are not for sale anyways. All you end up getting is a priceless/not for sale automobile that you can own and tune. The worst selling aspect to any GT is GT1, in which all cars you've won sell for only 10K Cr, and then 12K Cr if you take it to the correct tuner.
I'm sorry I didn't have much to contribute about concept cars and race series for them in this thread. Lo siento. Gomen nasai. All I can say is that I hope Concept Car races don't end up being as challenging as the Suzuki concept car One-Make race series. It was too challenging with