As I have exactly that revolver, I always notice them when I'm at gun shows, and I definitely go to my fair share of gun shows.
I would say that here in the fall of 2013, the market for a 686 of that vintage is holding steady at it's all-time high. At a gun show, where the market is active and the prices are negotiated all day long and VERY few people pay (or expect to pay) what the tag might say, I think your idea of $625 is right in the proper place.
Over the last handful of years, I've seen prices on used pre-lock 686 revolvers run between $500 and $650 depending on condition, features and different dealers' idea of price.
The price you saw tagged at $755 "may" be a bit out of line, but I don't think it's all too far out of line, because I expect gun shops to have a higher tag on most firearms than we'll see at gun shows.
I think your $625 price is a fine offer but I wouldn't be surprised if the gun shop elects to hold it and grab a bit more for it. There are -MANY- who would only ever buy from a shop and never in a private sale or gun show, and they will pay a premium to do that. (and for most of them, they either don't know or simply don't care that they are paying a bit more)
If I were in your shoes, I'd be visiting a gun show, or a series of them. You are also less likely to pay sales tax at a show.