Here's what has surprised me.
Bud's Gunshop sells the 617 Plus 4-inch or 6-inch for $670, delivered. (No relationship with Bud's but they are a good benchmark.)
The average price trend in my 617P 4-inch track is almost as much as Bud's price for a new one. Here's the track:
Smith & Wesson 617 Plus Revolver in .22, Long Rifle 4-in Bbl. Bluebook of Gun Values (31st Edition) indicates between $326 in 95% condition and $631 in 100% condition for this gun. I have 33 recorded prices for this model gun from $350 to $795 over a period of 6 Yrs 8 Mos ending April 29, 2010. The average price as of that date was $499. 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 $940.
Here's how the data charts out. Keep in mind the trend is for the average price. Better examples would command higher prices. So as of April 2010, the average price trend is at about $620.
Here's the chart for the 6-in 617P.
In my opinion, if you can find a 95% or better used 617P for around $600, you're doing OK. New ones under $700 would be a reasonable price, too.