If you want to use a single fabric on the back, you should get a minimum of 4 yards (assuming regular width) or 2 yards of extra wide fabric. I would buy at least 1/2-1 yd extra just in case (doesn't hurt to have a little extra).
In the past, I've always used one fabric for the back. I'm going to start piecing the backs with 10" squares in similar colors to the front, just in a random pattern. That way, I can use up some of my older stash and make room for some new stuff! Some of the prints in my stash have been used on several projects. Time for something different! :)
If I remember correctly, there is a yardage table on www.quiltville.com, linked on the right side of the page towards the bottom.