When you think about it, if you're doing a large quilt you might need about 9 yards for the backing, and if you do borders out of that fabric and some of the blocks too, you're probably talking close to a whole 15-yard bolt anyway.
I'm wondering if there is an un-met need among quilters for full bolts? It would be easy for a store to deeply discount a full bolt, if it's special ordered for a customer, there's little handling involved (you don't have to measure a full bolt), and the store doesn't have to "hold" the fabric, just move it in and right out again.
It would be nice to have the option to use a fabric that you *love* for the backing, if it's a quilt that you're going to keep for yourself, or give as an heirloom gift, right? Instead of settling. (If the price is right for your budget, of course!)