Four hundred dollars is a fair price for a 66. More than likely you are not going to find a real bargain on a gun like that unless you happen to be in the right place at the right time--people know what those guns are worth.
If the gun you are looking at is pinned and recessed, it is worth a little more. I did not say it is necessarily better, but there are people like me who refuse to buy a S&W unless it has a pinned barrel, and recessed chambers if a magnum. Why? Just 'cause.
I need a good 66, and I would pay $400 for one that was in good shape. Look for flame cutting on the top-strap and general looseness, both indications that someone has gotten carried away and shot too many hot magnum reloads.
Just look around and see what a new gun of this quality (if one is available
) would cost.