SCS&W places the Model 66 NIB at $375. These prices are 5 years old now, and the K frame .357magnum is no more. Therefore, expect the market price to be around $400-420. This price is for a true NIB gun though, not a LNIB gun.
If you are planning on shooting this gun, there is no reason to purchase a stainless shooter gun that grades greater than excellent. That places the SCS&W price at $285. add a bit to compensate for inflation over time, and the percieved rarity of the K frame magnum
, and you get $300. If it's a P&R gun, that's a good thing, so toss in another $25.
That would mean my limit for a shooter Model 66 in excellent shape, P&R would be $325. I might go $350 for a friend in need, if he had a couple of boxes of ammo and a nice holster. Your buddy has a Model 66 that is LNIB. I wonder what he will take for it?