I just checked flannel sheet sets on Walmart.com and you get a flat and a fitted set (full size) for $30. That would be enough flannel to back two quilts. You say it is a twin size quilt but without the actual dimensions of it, it is hard to guess how much yardage you would need. Assume the top is 71x96. If you bought wide flannel at Thousands of bolts which is probably the least expensive I have found it, you would need at least two yards (which gives you 72" x 108", probably more than enough to back a small quilt) and that would run you $28 plus shipping. Both of these options are less expensive than buying standard width flannel and piecing it. Even if you could find flannel for $5 per yard you would need to buy close to 6 yards (probably more to give you the needed wiggle room for the back)