The Six Series is a great line revolver, and folks who know them love them. I keep a stainless Security Six 6" by my bedside, matter of fact, althought it may be losing that job to one of my S&W K frames.
That said, for a blued Service Six, 4" model, in near excellent condition like that, as a buyer, I would expect to pay between $400-$475 in a private transaction, and probably close to $475-$550 in a retail setting. I would strongly consider buying it, as the Service Six doesn't seem to be as proliffic as the Security Six, and should show some appreciation in value over the coming years. They won't be like the Colt snake guns, but I can see them becoming more desireable to collectors.