[Stealthee]

Supply and demand. They only make about 100 Ariel Atoms a year. There are only 1000 X Bows produced per year. There is also a lot of engineering that went into them and the fact is, people pay the asking price, so why no ask it?

As far as size:
The Atom comes in at 11' 2" (3.41m) long and 5' 11" (1.798m) wide.
The X-Bow measures 12' 3" (3.73 m) long and 3' 11" (1.19 m) wide
The Miata based Exocet measures 11' (3.35 m) long and 5' 8" (1.73 m) wide

So your concern with the car based exocars being too large is unfounded. A good friend of mine just built a Exocet and it is quite awesome.

One thing you need to concern yourself about is legality. Not all "Go Karts" are street legal and every state/country.