FWIW, you'll generally pay a lot more for a converted rifle. The conversion process itself is relatively straight forward -- you can accomplish it in an afternoon in your garage using standard tools (drill, Dremel and a screw driver). Depending on how fancy you get with option, expect to spend $100-$150 in parts...you can use that as a guide to compare the purchase price for an already converted rifle (the unconverted is selling for $370 +S/H + FFL on GilbertsGuns.com)
Jim