Auto body shop rates in your area are probably about $60 per hour, figure 3 hours to clean the old metal, fixture the subframes and weld, then undercoat the work, throw in some $ for welding wire and undercoating materials and I can see $200 at a regular body shop. You might check with some custom muffler/exhaust shops and see if they can do better, don't go to the Meineke's or Midas shops, they don't like to do stuff that's out of the ordinary, look for a custom exhaust shop. I've had Taurus SHO subframes welded on by exhaust shops, these fellows are usually pretty experienced welders, then throw in a can or two of spray undercoat from Vatozone and call it done.