$500 is a great price for this model right now, especially if it checks out well. Here's my track on the 7-shooter version, the Bluebook of Gun Values puts its value $35 above the 6-shooter.
Smith & Wesson 686 Plus Revolver in .357 Magnum, .38 Special, Plus P. 2-1/2 in Bbl. Bluebook of Gun Values (31st Edition) indicates between $375 in 95% condition and $650 in 100% condition for this gun. I have 81 recorded prices for this model gun from $350 to $900 over a period of 6 Yrs 8 Mos ending February 16, 2011. The average price as of that date was $537. No refurbs, no refinishes, regular sights. All represented as 95% or better. No distinction for engineering revision, if any, no special editions, no distinctions for box, papers, tools. Current or last MSRP is $964.
Here's how the data chart out. Note the trend in price is about $620 and the monthly count trend is increasing. This is a change from when the 2-1/2 incher was hard to find (see before April 2008). Prices were attractive, so evidently, folks who had them decided to ride the wave.