Shoo,
I have had an M&P 9 for about a year and a half, and really liked it. So much that after getting a 4566, I had gotten the urge to get an M&P 45. I too was worried about the cost, and in expanding my shooters, have become a real bottom feeder on prices for used guns. But I found that on Gunbroker and even thru a couple of dealers that new M&P's are going for under $500, shipped, and in many cases under $400. So I finally got one last week, delivered today, and with the rebate (which ends tomorrow), it will come to $415 delivered. Had to go thru 3 auctions, and then I was bid up about $30 more than the recent bids, but with the rebate ending I could not resist. To top it off, it came with Nightsights.
AS luck would have it, just after I win one, several used M&P 45's popped up and sold this week for $350+/-, and one, almost new in the box, even went for $250.
In comparison, I paid $600 for the 9mm.
Good luck on your search.