The circle track stuff is much less expensive because of the volumes. There are lots and lots of these cars built and many piecs get standardized.
As far as building a race car goes, there are many levels of that as well, most are not cheap if you're talking about real race cars. My experience has been that if you are really going racing you will burn through money much faster than doing it for the street.
Just like cars, there is shade tree mechanic stuff and real high end. For example - multi-disc clutches, gun drilled axles, real fuel cells (can/kevlar bladder/foam/rollover valve), etc. If you look at hard core racing pieces they will almost always be way more expensive than their street counterparts. ven if you look at road race vs. circle track you will generally see much more exotic stuff on the road racers.
We're just about to start building a track only car with our Level II Suspension/Brakes on it so we can seriously abuse it and also keep it safe. Much of what we've done is taken direct from current road race level product and technology.
John Buscema
XV Motorsports
www.xvmotorsports.com