I use quilt shop quality fabrics that I happen to have bought on sale for the backs of my quilts. I want the back to look good and be as durable as the front with the same amount of shrinkage etc. I have been very fortunate to have been able to purchase some large pieces of very good fabrics from $1 to $3 per yard. Whenever I can catch a sale like that, I stock up and buy the 6-12 yard pieces in colors I use most.
My backs may not necessarily "match" my tops but I have been able to geth them to "go" well with them.