For starters, prices vary a great deal by model type and iteration. Also, as the M&P (Military and Police) is aimed at that market, the lack of federal excise tax for Law Enforcement personnel also brings the price down for some buyers.  A more universal reason is that the Shield is considerably smaller than many other handguns, meaning that less material is used in its construction. Coupled with the polymer body being cheaper to make than a metal body, this further lowers the price.