Anecdotally, used M6 in decent condition were +/-$1000 for many years, including during the early 2000s. Those in lesser condition were less, and rarer ones (say the M6 with greater vf magnification introduced in the late 90s) would sell for more. This is the M6 classic, not the TTL.
I ran the used dept of a camera store in 2009-2011 time frame, and they were about the same price at that time, too. Some dealers would sell premium condition units for say $1295 or so, but many were sold by individuals on ebay, and websites like this one, for roughly $1000. I bought and sold many.