Chris,
I think you really need to provide more detail for anyone to help. I probably wouldn't buy gen 1s at any price at this point. I consider gen 2 and 3 roughly the same (I own the 'pro' versions of both) and gen 4 is different with Ecotech's latest effort to reduce the 'spotlighting' than can be problematic with puck style LEDs in my opinion.
I would probably pay 200-300 for non-pro gen 2 or 3 lights, 300-400 for pro versions of gen 2 or 3 lights and maybe 500-600 for gen 4s with minimal use. Of course, everything is worth only what someone is willing to pay for it. I would check out the equipment sale forum here. Even if you don't buy through it, you should get a pretty good sense of what a particular light typically sells for.
Matt