That's the problem with digital cameras - they lose resale value rather quickly as newer ones come to market. I had a D70 sitting around that I rarely used and decided to sell it. I only got $150 for it, including batteries, cables etc. I had similar misgivings as it was after all a fully functional camera but in the end decided to sell to simplify my kit.
The resale prices you are seeing are not surprising since for not too much more one can buy a brand new, current low-end camera with improved features. For instance, on B&H right now you can get a brand new D3100 with 18-50mm VR lens for less than $500. Given that, how much should a used D200 be worth?
I agree with the suggestions of others that is good to have a backup camera.