The value is going to depend on your location. If you're in one of the handgun law strangled states like California, prices are inherently higher than the "free" states.
Value will also be highly dependant on whether or not the person that buys it values the porting... that's going to be the biggest factor.
There seems to be consensus/controversy out there that many hold... that Mag-na-porting decreases the value because it renders the gun non factory in appearance, and many also feel that any perceived reduction in recoil/muzzle flip is offset by the increase in noise.
I tend to disagree, my experience with a Mag-na-ported long barrelled revolver (and it happens to be a M29-2) is a positive add-on... but you'll probably have 10 people saying it ruins the gun for every person like me that thinks it's a plus.
The other pluses is that being a -2, it is a "pinned & recessed" gun, the 29-3 eliminated the pinned barrel and recessed cylinder, which many people disliked happening.
So... ballpark value given the variables, I'll guess $600-700.