I don't know too much about the cost of a new caterham when it's all said and done, and all that is required to put one together. But having just bought a used one I can say I must be close to the cost of a new one with the engine rebuild, replacement parts, etc. And the car still has a lot of worn out bushings, cracked body parts, pitted windshield, bent suspension parts, rust, non-functioning gauges, and who knows what else. My decision was made largely, though, on the basis of it being registered in California already, and that was nice to have. My only advice would be that if you buy a used one, make sure it has been well cared for, or it may not cost that much less by the time you get it back in shape.
Justin