I'm no expert but just off the cuff I would say that the .357sig would probably bring a bit more than average used M&P prices but the smaller pool of buyers would require patience. Many who wanted the .357sig caliber just bought the barrel and put it in their .40 while others (like me) want the whole gun. I have a compact version and just bought another for my Dad. It's a LNIB .357c marked as such with original box and a S&W .40 cal barrel included. $425 and I thought that was a very good price.

I'm fortunate that I hand load so the ammo costs no more than other stuff to load but I know the shelf prices can be high...that pushes some folks away. Watch the auction sites to get an idea of what things are actually selling for. Good luck.