This a complicated question. I'll try to answer it without too much detail, but I'll probably fail, cause detail is what I do best.
The most common barrel lengths for these guns are 4" and 6"; in my experience, 4" guns are slightly easier to sell, but go for about the same amount. The usual configuration for these guns is a red ramp front and white outline rear sight. In this configuration, in VG/Exc condition with 90-95% finish, a typical M686 will generally sell for $450-$525, with a nickel M586 perhaps $25 lower and a blue M586 another $25 less. FWIW a gun in low condition would probably be less, but I simply don't see very many of them.
HOWEVER...
Here's where it gets more complicated.
Original matching boxes and docs add value to any of these guns- typically $50ish.
The traditional forged lockwork and hammer-mounted firing pin are found on the M586 no-dash through -5* (more on the -5 below...) and M686 no-dash through -4. The -4 versions are considered the most desirable because they are the only ones with forged lockwork AND a topstrap factory-drilled and tapped for a scope base. Also, the 7-shot M686 "Plus" began to appear during the -4 run. All of the -4's command higher prices amongst knowledgeable enthusiasts, with the M686-4 Plus at the top of the proverbial heap. These guns may push $600 in VG/Exc condition, even without the box, etc.
The M586-6 and M686-5 were the first with MIM lockwork, a floating frame-mounted firing pin, and flat-faced hammer. These guns sell for slightly less than the older ones. Later versions with "The Lock" are even lower on the totem pole.
The M586 was discontinued almost immediately after S&W started using MIM parts, and very few 7-shot models were made. Consequently, roughly 95% of M586s are 6-shot with forged lockwork, so the absence of MIM does not mean much on a M586, but 7-shot M586s are downright rare and generally sell for BIG premiums!
8-3/8" versions of both guns exist, sometimes with an adjustable
front sight for long-range Silhouette competition. These guns are technically worth a premium but seem hard to sell due to their prodigious size and weight, so I've heard of people getting a bargain on occasion. OTOH most of the ones I've seen have been in barely-fired 98%+ condition with the original box and docs, and priced accordingly.
I have seen a few(!) LE trade-in M586s with very rough bluing trade in the low $300 range, but these guns seem scarce. OTOH it takes a LOT to truly ruin the finish on a M686, so it's less common to see them take a big value hit for bad finish.
About 4 years ago, an LGS had a number of LE trade-in M686s for $375 IIRC. They looked decent, but most had nasty scratches, and some klutzy armorer had scrawled big inventory numbers on the sideplates, apparently while drunk.
They
still sold fairly quickly.
The 2-1/2" and 3" M686s sell for a $75-$125 premium in my area due to their desirability for CCW. S&W fans in other areas have reported lower premiums; YMMV. OTOH 2-1/2" M586s are downright rare and generally sell for big $$$, like the 7-shot guns.
The M586-5 was a special run for the Brazilian gov't chambered in
.38 Special only. IIRC they were all 4" with blue finish. A few contract overrun guns were sold on the US commercial market; these are sought after by collectors and usually sell for big money.
The M686 CS-1 was a special edition built for the US Customs Service in the 1980s; IIRC these were RB guns with 3" or 4" barrels and bead-blasted stainless finish. They were assembled with extra precision and feature factory trigger jobs. You know where I'm going with this- yup, another high-buck collectible.
S&W has also built a few 5" M686s, including a "Stocking Dealer Special" with an orange HiViz front sight and cocobolo finger-groove Combat stocks. Yes, more of the same...
Hope this helps, and doesn't just confuse you more.