Speaking personally, if your $1800 price is right, I'm amazed that a 4 year old camera can maintain that sort of used price level. I think it is a testament to the camera and how the market values that camera. I would not buy one used at that price, just because the warranty issues, etc. But that is a personal valuation of things.
(I am not following used prices so have no opinion on the accuracy of your price assessment although it sounds right to me)
I think the relatively higher drop in the D3 price reflects the fact that there is not as much "value" in that camera, relative to the D700, and the marketplace values it accordingly.
And obviously, "value" is in the eyes of the beholder and for some the added features and ruggedness of the D3 may be very critical. I'm just trying to assess the overall market's valuation, as reflected in used prices. Not trying to judge any one person's assessment of "value"- that is impossible or unwise, in my opinion.
I suspect a year from now the D700 price will drop commensurate with a D600 intro or something similar, which is inevitable. I could see it down near $1200 or so but that is just a guess, or maybe hope, since I'm not sure I wouldn't buy another if mine fails badly, given what appears to be on the horizon. I really need 8fps and that seems to be the one feature that may not appear in a near term replacement or alternative like a "D600" intended to be an economical FX offering.