I'm thinking about picking up a 5906.  I know there are a plethora of LE trade-ins out there, but I'm looking for one in excellent to like-new shape.  Doesn't need a box necessarily, and I already have plenty of mags.  What should I expect to pay?  I've been checking Gunbroker, anywhere else I should look?  Prefer the round trigger guard and Novak sights.

I'd say anywhere from $350 to $450 in the condition that you're looking for.

I paid $350 for this one about two years ago..

Nice, pretty darn good deal even for 2 years ago.  I have a line on a couple locally that look pretty nice, unfortunately they are around $500.  I would gladly pay $450 at this point, but $500 is pushing it.  The search continues.

I just got a pristine 5906 with novaks and rounded trigger guard last weekend for 479. I probably paid a little too much, but I'm happy.

Fair price considering the current market it seems

Well I picked up a 5906.  I still ended up paying $500, but the gun is in very nice shape with low round count (below 200).  It has the round trigger guard and Novak sights.  Also came with 2 Mec-Gar 17rd mags, 2 sets of grips, and 300rds of 147gr Federal Hydrashoks.  All and all I think it was a pretty good deal, especially considering one of the 5906's I was watching on Gunbroker just sold for almost $800.

Considering the hydra shocks are worth over 100 bucks alone ( at a conservative $25/50) you got a sub 400 dollar pistol- I'd say you did well!

Love my 5906.  Never carry it but love to shoot it.

I have a 5906 my dad gave me after CT decided the magazines were too dangerous for private citizens to own.  Mine was a former cop gun that my dad bought from the officer who carried it- so it looks like a well used police gun.  I would value it at about $350 in a private sale.

Arguments for a lower price:
- The 5906 was the most mass produced gun so they aren't rare by any means.
- They have no real 'collector' value like a 1086 or 4536.
- Support from S&W is pretty much gone. They'll fix your gun but parts are no longer being produced; once those on hand run out you're on your own.
- For people overly concerned with 'concealment' the heavy bulky 5906 doesn't carry as easily as some other more modern options.
- It's a lot of steel for the marginally adequate 9mm round.  Sig P320s, Glocks, and many others do it all for less weight, and if you want steel there are still better options that are lighter and thinner.
- Even a 5906 in poor condition can be easily cleaned up with a bead-blaster- the clean look you see might be from lack of use, or from refurbishment- it's difficult to tell.  

Arguments against a lower price:
- Used parts should be available for quite some time given how mass-produced the 5906 was.
- Good condition adds to the price, as does having the box and extras that came with the gun new.
- Magazines-a-plenty.

I would LOVE to find a good condition 4526, 4576, or 1026 (to go with my awesome 1076) but the value I give the guns, and don't get me wrong I love the 3rd gen Smiths, is substantially lower than what people are asking and getting for them.  For example, given all of the above, I would be willing to pay $500 or so for a good condition 4526, but they're asking $800 or more, and sometimes getting it.  

EDIT to add: My gun buying may be different than yours.  I don't own 'safe queens' and I don't collect.  I buy utilitarian- in other words I buy handguns that will fill a role for use; that may be for carry, practice/plinking, glovebox gun, etc.  I don't usually buy a gun just because I think it would be cool to own- I have to see it filling a gap.  Your buying may be different.

You have to weigh it all in your own head, and decide what you're willing to pay.

Also, here's my 5906.  Some, like me, prefer the square trigger and rear sight hood, and other hate those features.  Some of the later produced Smiths used MIM parts, so for me older would be better.  I put in a heavier recoil spring and keep it loaded with hot 147gr +Ps to get the most out of the 9mm.

I have NO issues carrying a 5906 with a 17rd mecgar mag.

I just make sure to take a good dump before I leave the house to help compensate for the what, extra pound of weight versus modern lighter carry pistols.