The price seems a little high, see this current thread:
8 3/8" 686
You can't compare a private or store sale to an auction, but $375 is a good deal for a used gun and $650 for a newish one is above market value. I'd guess about halfway between those two values is about right. The shorter barrels (2.5, 3 and 4") tend to be more sought after. Hope this is helpful.