It all depends on your location and the demand as to what the price should be. The main factor is condition, barrel length, and what goodies it has like the original box, tool kit, and original paperwork. The 4 inch 686 sell for a premium over the 6 inch and then the dash number if any also affect the price. So we need more information to help you with the offer.
I saw a 686 4 inch -1 here and I could have bought it for $475 out the door but it had rubber grips, no goodies, and had been shot a lot and had some rubs and a gouge on the right side of the barrel so I passed. If it had been mint with the goodies I would have paid $600 for it.