When you need to buy a car for a new race event, purchase it off ebay instead of buying it new. Look for a low cost that still has a good track record. Use it for a few races and pick up some wins, using restarts and flashbacks if you need to. The more wins a used vehicle earns, the higher price it will command on resale.
Once you've stopped using that car, likely from buying a better one in its class, post it to ebay with a Buy It Now value higher than what you paid for it. Just below new retail price should be good. Then set an opening bid just below what you paid for the car, and give it a few days on the listing - maybe 3 or 4.
You will get updates after each event on how the bidding is progressing, and you should turn a profit by the time it sells. If not, try again.