Contact Mike Orlen in MA. He can provide a slightly better cost than Briley and his work is highly respected in the industry, especially in the area of fine doubles. (You can find him on ShotgunWorld forum.)
My dad threaded my shotgun for choke tubes. In short, it takes a pretty good lathe and the right bit. Its a very easy job to screw up so I would suggest paying the extra money and sending it to a gunsmith.