Check with Botach on occasion also. They've been known to offer M&P mags for very reasonable prices when they have them in stock. I personally would not pay inflated prices for mags (or anything else) unless I was absolutely desperate. I know the wait is hard to deal with, but there are some signs that this frenzy is burning out to some degree; and it would not upset me a bit to see those who bought stuff in bulk with the intention of scalping end up taking a loss on their efforts.
For what its worth, a friend bought a brand-new Colt AR model 6900 flat top, no sights, one mag for just under $1100 from a gun store in the Kansas City area yesterday. He called me to ask if it was a good deal. I had to tell him that I didn't think he was getting a terribly bad price, as the Colt name alone commanded a little higher price than a comparable Bushmaster, etc. Back in December, we had looked at a no-name brand M-4 style carbine that a shop was selling for $1500 a pop and on back order to boot. Quantico Tactical is back to offering certain M&P 15s to LE/Military for under $900. Assuming we don't hit any more emotion-fueled bumps in the road for the rest of the year, I think the prices are going to come down and the availability will return.