There out there you just have to be persistant. check you local shops used counters & talk with the shop owners. Let them know what you looking for. If thewy know they can sell a used weapon they are more likely to take that model in on trade.
It took me a year of checking the 5 local shops. Then one day I walked in a place and found A premier FA .454 4 3/4 barrel, ported, trigger stop for $900. I couldn't be happier with it.
I think anywere around 50% of new cost is fair. I also believe a lot of traded in casulls don't have many rounds put through them. The deals are out there you just need to look.