My 2 cents
We all know that a car is worth what someone is prepared to pay, not what we as owners think it´s worth.
Personally I believe that condition is everything. OK, if a 2.5 in the same condition as a 3.0 CS or Euro spec CSi, the latter will normally be more expensive.
We all know that the driveline is the easy bit. That leaves the body and interior. The body can be repaired very differently by different people. A non repaired car will always be more sought after, but seriously good bodywork coupled with a first class paintwork is nearly as good. The interior is the difficult bit on the E9s as so few parts are available, besides carpets, and recently headliners for cars with and w/o sunroof.
Top quality chroming is very, very expensive. It will set you back several thousand dollars, at least in Europe, once the underlying polishing work is done perfectly, plus the 3 dips are nowadays extremely expensive.
Top CSi cars in Europe, by dealers, have asking prices of around 40.000 EUR or over 50.000 USD. The big market is Germany, of course. Trim off 10% perhaps and you get an idea of what they cost at dealers. Private sales, I imagine, result in lower prices. Rough cars don´t sell at all, as the restoration costs will far exceed market prices.