Yeah, I'm not sure how the economics of that work out... for example, in LeMons, you can't spend more than $500 on the car, not counting safety equipment. Granted, safety equipment makes up the vast majority of your investment, especially so if you're paying someone labor to build the cage.
Maybe there are folks out there that just want a turn-key experience, but I'm not sure why those guys would be interested in shitbox racing... where the real appeal is... well, in building and racing your own shitbox.