Joann Fabrics just had a 70% off rolled batting coupon (don't have to buy the whole roll). I sometimes think they mark the prices up so customers are forced to use the coupons to buy anything. Keep watching their ads - go to the web site - download the app. Bide your time and when it's super cheap buy as much as you can.
That being said, flannel makes a nice batting and a friend of mine always uses fleece on the back with no batting.