The one Pachmyar I tried in my P-220 didn't work. Wouldn't feed. They are apparently not made by Mec-Gar, but by some US company.
(This topic has been discussed on the SIG list in recent days, and there has been mixed results with the Pachmyar.)
If I were you, I'd stick with factory mags (or Mec-Gar, since Mec-Gar builds Sig's mags). Check CDNN (in Gun List or Shotgun News); they generally have Sig mags for quite a bit below list, or go to a gun show.
THe price you cited was NOT particularly good.