I pay $2.75 or $2.95 at my LQS. I wouldn't pay more than that. I found the inexpensive ones at the stores are cheap fabric. I learned my lesson after making a few runners. They don't feel as good as the better quality ones.
I bought 40 of them at my LQS for a quilt when they had them on clearance for $1.00! I was so thrilled. Saved me tons of money!
You can buy them cheaper than eBay on www.fatquartershop.com.