Depends on the level of barrel you want installed. I am having a gun built and if I remember correctly the barrel with instalation was about $500 of the total price of the build (I might be WAY wrong). I would expect about $200 plus the price of the barrel. A take off barrel or lower grade barrel will be cheaper, a top-of-the-line barrel will be more; just depends on what you want. Again, I'm not a smith so I might be way off.
As far as gunsmiths in Montana...where are you located? I know Kirby (WELL known on this board) is in Ft. Shaw (near Great Falls) and Young's gunsmithing in Tracy (also near Gt Falls) would both do a great job. I'm sure there are probably more good smiths around, these are just two I know of.
Genises 27-3: Now therefore take, I pray thee, thy weapons, thy quiver and thy bow, and go out to the field, and take me some venison...