Prices seem to vary on these depending on whether it has a synthetic stock or wood stock and the caliber the barrel is. On the auctions these rifles have been running at around $450 and up, depending on condition. Of course, whatever someone is willing to pay will determine the price. Myself, unless it was in a scarce caliber, I wouldn't pay much more than $500.
Pardon the nitpicking, but these aren't SIG Sauer rifles. They are pure SIG Swiss rifles and have no relationship with Sauer other than the company that imported them. The SIG 970 SHR is still in production and is a relatively popular rifle in some parts of Europe and of course Switzerland. Unfortunately SIGArms USA or whatever they are calling themselves now, discontinued importing these rifles several years ago. I believe they started importing them in 1997 or 1998, but need to check that. The American SIG Sauer company dropped support of these rifles a long time ago and parts and service are very scarce.
I don't know the reasons that importation and support were dropped and have tried on several occasions to get information from the American company, but they aren't giving out any details. In the period of 2000-2005 SIG in Switzerland was going through business adjustments and changes and the company was bought by two Germans, but production for Europe has continued and I don't know the ownership situation at this time.
The SIG SHR 970 is an interesting rifle because of the interchangeable barrels and manner of production. They are of very high quality even though they are mass produced, but honestly they are rather plain in finish. The ones I have are extremely accurate and they fit me well. Extra barrels are very scarce in the USA and only a limited selection of calibers was imported into the USA. The European selection was much better and had some really interesting metric calibers. Unfortunately, they are not being imported into the USA.
Another rifle based on the same action is the SIG STR 970 which originally stood for SIG Swiss Target Rifle, but of course soon morphed into Swiss Tactical Rifle. I don't know if the original European version ever actually was imported into the States, but special stocks and heavy fluted barrels were available for a short time here in the USA and so occasional you will see a SIG 970 tactical or sniper on the auctions that are made up on the actions. The originals had an integral muzzle brake and the made up ones won't have this.