I just found this place, and their prices are terrific. They have quilt backings for $2.75/yd-$4.75/yd, and all sorts of other bargains. It's sort of a pain to have to go through each color family, but it you have the time, it's well worth the work. Alas, they don't have free postage at those prices, but even then I believe I averaged $3.60/yd. Not half bad, eh?
http://www.quiltingcotton.com/