I just shipped a gun just like this to a buyer off of gunnutz today.
It is a 1978 S&W .44 Magnum Model 29-2 with 8 3/8 inch barrel and original checked Goncalo Alves grips - looks EXACTLY like the one in your pictures. It had very, very few rounds through it and was in mint shape - even had the wooden box like the one shown.
I sold it with two Bianchi leather holsters, one under the arm, one on the belt for a total of $1050 to a guy who knew exactly what he wanted. I actually responded to his ad re looking for one and we ended up cutting the deal - it was important for me to know it was going to a good home (it wasn't really for sale, but I ended up selling it to him).
Thus, if I adjust the $1050 by knocking off shipping costs, the value of the two holsters and the fact that my buyer was set on that particular gun, I'd say that about $800 is a reasonably fair estimation of this pistol's value.
By the way, it is one hell of a gun in all respects, and with the long barrel and all that power I was able to pull off shots that made me a bit of a local legend around here.
Feel free to message me for any further info about any of this, including the ownership experience...