I am really bad at bargain shopping and typically buy whatever I want when I want it...with that said, I have never used sheets for backing. My LQS will sell me discontinued bolts or bolts that just aren't selling at $5.00/yd and, if it is versatile for backing than I will buy the bolt. I buy, end of bolt, fabric at discounts, too.
Someone should start a swap for 1/yd fabrics to help recycle everyone's stash that no longer appeals. That would be a hoot!