My backing is usually 45" fabric, many times I'll just use what I have and then add some blocks to it if I am short, it adds a nice secondary look to a quilt. I use the 108" when iv'e done a king size, just as it is cheaper.
also, for my backing, when the local quilt shops have their end of year sales I usually buy the fabric on the clearance rack for backing. nothing like $4.5 a yard for top notch fabric.