in my opinion, this is really depend on the time piece which you looking at/for. Different pc has different used value really. Like Chris was saying, a lot of factors (that affecting the used price) come in when come to a used time.. honestly, i'm not an expert here who can tell what watch has better value or not... the value come in if there is any demand, if higher demand (less in supply/production), better used value. Eg: Panerai. Not many people appreciate Panerai but i have to admit that it's the brand that growing fastest i have seen so far. THe used Panerai's price holding very well because still a lot of demand out there.
Understand you were trying to look for a "standard" or trying to find out a reasonable price in general for all used pc but in reality, it's not really that easy to achieve this.
in my opinion/observation, usually the used shop will sell 10% to 20% lower then the after discount retail price for a good condition pc, but then again, it's depend on that pc's demand
Eg: compare Big Pilot and any JLC (similar complication), BP fetch better used price anytime because of it's demand. Used JLC maybe 40% to 50% from retail price. But, this is not definite, maybe one day JLC will fetch better used price in future if JLC have higher demand.
I think that's the reason why Chris was saying, do some homework before you buy it
Usually, this is what i will do, if i wanted to buy a used pc, i will check what's the brand new prices and discount i can get from AD, then compare. if the price is not a lot different, i will take brand new from AD.
Also, always buy from a trusted used shop to make sure your purchase has no problem, not just the authentication but the quality of the watch you are about to purchase, a honest shop will tell you the history of the time pc and advice if you need service or has this pc been service before; etc..
hope this help..
scott