A CLEAN 686 should run between $550 and $700 depending.
The 686-4 is considered to be the Cream of the Crop, and can bring prices on the high end of the scale.
On the other hand the 6" normally is not quite as desirable as a 4" or 3".
In my area, atleast before Oboobs Gun Sale began a 6" would have been $550 - $600 in clean shape.