My oldest one is a youngster compared to most of the guns mentioned here so far, a Dan Wesson .44 Mag, vintage about 1980 or so, it's a very low serial number, so it was made soon after they introduced it.
Until recently, I had an S&W "Triple Lock" in horrible condition that a friend gave me as a birthday present. It had no finish left on it, it did cock, but wouldn't have been able to actually fire as the hammer barely moved by itself when the trigger was pulled, there was severe pitting and huge amounts of rust inside the bore and internals, and the aftermarket grips were glued back together after a drop, I guess. I don't know where he got it, but he had it for 20+ years. I asked several people what it was worth, and nobody thought it was worth much of anything. A couple of gun shops offered me $25 for it I was thinking about maybe cleaning it up and making some sort of display out of it, but some old guy offered me $200 for it, off it went. The other guys at the gunshop thought he was nuts for paying that much, but he seemed happy to get it. $200 was better to me than a very nasty looking old gun that wasn't ever going to be shootable.