When I plan to make a quilt, I also plan (i.e. purchase extra) for the backing. Often, I piece the remaining scraps from the quilt top and incorporate into coordinating fabric for the backing. I agree, it's a costly part of the process but I prefer gifted quilts to have continuity on the front/back.
However, I do have a very scrappy project that I work on in-between others and when I'm finished I will piece a very random, almost large crazy quilt, backing of "what was I thinking" fabrics. Perhaps I should get out of my box and liberate my quilt backs more often! It isn't necessary to perfectly coordinate but it sure does present a quilt nicely!
Isn't it wonderful to have so many choices?