That is a good question, but a difficult question, in my experince. Just tak a look at this recent thread:viewtopic.php?f=14&t=102301
You will get varying opinions based on, if you consider yourself as the "seller" or the "buyer".
I assume you payed, in the "ball park" of, about $3500-$4500. (New, U.S.)
I've been in the market in the past and seen used sets between $2100-$3200. (pretty good condition)
But prepare you self for some pretty "low-ball" offers. (but as fortune has it, you don't have to accept those offers