Yes, I have seen some great deals in the local papers and wonder the same thing. I suspect it is just old fashioned supply and demand. There are probably a lot of units and/or components in the supply chain and SIG is simply trying to move them.
In fact it may be because Sig is (supposedly) discontinuing the line. I say supposedly because they have done this at least once before, and then restarted production. But any way you slice it, it's a great price for the gun.
...third option - Sig built a truckload for the US Army Armor division...is it possible they're switching sidearms, and Sig will have an abundance of these pistols around?
Polymer is dirt cheap compared to steel or aluminum. I think a Glock frame cost less than 10 dollars or something to that effect. I bet all polymer guns could sell for half their current retail price and the gun makers could still make a heft profit.