I agree with Aroc, that most used high end gear, including cables, go for around 50% of retail. (That is how I upgraded my entire system, (including, conincidentally, my used Cardas Golden Reference cables), in the past 18 months or so.) The minute a piece of equipment is taken out of the box and used, the price drops in half. (Even equipment that is new in the box drops a third in price!)
The only exceptions to this that I have seen are:
A) Items that have just come out (meaning limited supply), and have gotten great reviews. (An example would be the Esoteric DV-50 universal player, but even that is coming down in price now that there seem to be enough units to pop up on Audiogon on a regular basis.)
B) Items from companies that operate direct from the manufacturer, (and have great reviews also.) (Examples of this are the Resolution Audio Opus 21 CD player, and the Teres line of turntables. Both companies are well thought of, and since their products are more reasonably priced to begin with, (i.e. No dealer markup), the price seems to stay up in the 60-70% range, because most consumers (including myself), hang onto them longer.
Yep, you should always try to wait and see what used piece of gear turns up on Audiogon. You can save a small fortune!