I use simple, cheap broadcloth for most of my backs. It's $2.99/yd at Hobby Lobby, and with a 40% coupon, it's under $2/yd, even with tax.
If I want something just a little special for my backs, I buy 108" either at fabric.com or Thousands of Bolts. I don't go over $12 or $13/yd on the 108" backing.