Welcome to the forums from the Wiregrass! The best reference on recent sales is the completed auctions on gunbroker.com. The only example I could find is a .38 Special version of this gun for the Conrail police that was new in box. It is fairly rare and sold for $805. I'd say $700 is tops for your gun. But you never know until you put it out there for sale.
In my area, a 681 like you have would sell in the $550-650 range, more would be doubtful. I dont see the 681 very often, compared to the more common 686. A year or 2 ago I saw one listed at $425 and thought that was a smoking deal. The ad was down within an hour which I am assuming is because it sold