You can spend as much or as little as you want. Being the fastest car there is lots more than being .2 or .3 under. Being in A or B is way more than being in L or M. Buying a running car is way less than building. Pickups and open trailers are cheaper than Renegades and Gold Rushes. If it helps, everyone pays the same for entry fees.

You could buy a used pickup, open trailer and a running FWD stocker for 20K, maybe. Or you could spend 750K and have nice stuff and a fast car. Pick your poison.