Thanks again, and my apologies as well, Lindaschipper.
To bring it back in line with your initial post, I will say that the white fabric was a thrift store find at about $4/m - sadly, when I got it home I discovered that there was a small hole ripped through the centre of all of the layers (they'd nicely folded the fabric so I didn't see it in my cursory inspection at the store), so I ended cutting the entire length in half parallel to the selvedges. That being said, I really like it anyway - it's such a fun, quirky print. The navy solid was stolen from another intended project and had been sitting in my stash for years. I've been using it as a border on these quilts as well so there was some connection between the top and the back. It really makes sense to me to try to find sales on lengths or to creatively source backing fabric - when I've ended up spending more on the backing than I have on the top, I feel a bit let down to think it probably will never be looked at. I do agree that "ugly" fabric doesn't really belong there, though, whether it's an unattractive print or poor quality - the backing is still an important piece of the quilt! :-)