639 is among the more loved of all the 2nd Gens -- with 2nd Gens (generally) being without argument the least popular of all the 1-2-3rd Gens. The 639 also occupies a unique place for the fan of S&W pistols because it is one of extremely few production models that combine these features:

Single stack 9mm
Full size
Steel frame

The only models that fit those criteria are all scarce. A steel frame 39 is one of only about 900 ever made, worth a couple thousand bucks. A 539 is the blued steel version of the 639, scarcely made -- hard to find and collectible. The 3906 is the 3rd Gen version and it was made in very short supply and can be difficult to track down. The only other that fits the criteria is the Performance Center Model 952, and those were hand-built elite target guns, admission is $2000 and up.

That makes the 639 a desirable 2nd Gen when many 2nd Gens are more "ahhh, okay."

If you are talking mint shape, original box and goodies, $750 would be a fair price for a buy in. Solid shape with very little wear and original box? Market is probably floating around $600.

Shooter grade, looks decent but no extras and maybe missing the stocks? Solid buy at $400-$450. (Good news is finding stocks is extremely easy)

My estimates are for fixed rear sights and the screaming 80's hooked trigger guard. Adjustable rear sight is worth a bit less (no matter what SCSW says) and the rounded trigger guard is definitely worth more.

Local markets vary -- and for sure, not everyone who offers one for sale knows that they aren't always easy to find. The above is merely my opinion, but if anyone wants to tell me I'm nuts (I may be ) make sure you can actually FIND and BUY it, because they aren't spotted as often as most 1st Gens and 3rd Gens.