Matt,
It all depends on the condition of the lens and whether it is in original packaging with hood, etc. This is a very rare lens, probably the rarest of all Contax lenses, with possible exception to the 500/f4 Mirotar.
If the lens is in "Like New In Box", just as it was when purchased new (with all caps and hoods, papers, etc.), then I would give the seller whatever he asks for it. Period. If this is the case, and you decide to pass on it, please let me know as I will buy such a lens. By the way, the list price for this lens when sold in 1996 was about US$7500.
If the lens is in Mint condition, but without hood or box etc., then the value is roughly US$4500. With the anniversay hood and original packaging, the current value is about US$5600.
Below mint condition, the value will drop quickly according to how heavily it was worn. However, even a beat-up "bargain" condition of this lens is still valued at about US$2000 as long as the optics are good and it is functioning well. If something is broken or needs any serious repair, it may be worth much much less (if anything), since many spare parts are no longer available from Zeiss.
So, in general, the lens is worth a great deal to collectors when it is in LN condition. Otherwise, the value drops considerably. For a "daily use" lens, it will someday require cleaning, calibration and/or repair which will no longer be possible in the near future from Zeiss, so this should be taken into consideration.
David