I've never used a wide back fabric. I either use pieces, usually of what's in the top, or if I'm going to use one fabric, I piece that into a wide enough piece.
I check sale bolts at shops, plus when I go to a retreat each year, at the end, the owner has sale bolts that are $3/yd. I can usually find things that might not be pefect, but certainly work! With most patterns, if you want to, you can match them up so it's hard to even find that seam.