"The only limitation would be that purchases couldn't exceed $700 billion *outstanding at any one time.*"

http://online.wsj.com/article/SB122191819568460053.html

Their task is to buy the worst of the worst, and negotiate with bankers on the price. The worst of the worst would be worth approximately zero, right? But we'll pay anyway. I can only assume that we don't get to mark them down further or write them off, then rush in for more.