Quiltville has a table to help you figure out what yardage you need.
http://quiltville.com/backingyardage.shtml
There are a few sites that have 108" wide for $5-6/yd. I don't know what the quality is. I do plan to try some eventually. I just have everything I need for backings at the moment. If I remember correctly, Marshall Dry Goods and Whittles fabrics were among those in that price range. I just purchased regular width at $3.95/yd at Thousands of Bolts because I wanted a certain print for the back. Boy are they quick too. I got my UPS tracking number less than 4 hrs after I ordered!