6.5 Mag Ammo is like Hen's Teeth to find, unless you piece them together yourself.
The 600's and 660's are going at a premium on the Gun Auctions lately. Why that is I have no clue. I would suppose that if you found it in a BDL it would be worth a pretty good buck at retail prices, depending on condition. If you found one NIB, alot more. With the longer Barrel it would be much more suited for those particular calibers.
I'd snap it up if I were you.