I love reading how many people make quilts to give to others!! I too make a number of quilts to give to others (dislike "charity quilt' label). I buy flannel sheet sets on sale (got king sets from BB&B last year for less than $25) and that will line, or back five or six quilts. I shop Thrift Stores and have made fantastic purchases. If you have a Thrift Store near you, get to know the people and ask them to call you when they get in fabric. Some stores will do that and I've gotten complete stashes several times. Usually less than $1.00 a yard. Check there for sheets too.
There is a difference between "cheap" fabric and "sale/discount" fabric.
God bless each of you for taking your time, talent and fabrics to brighten someone's life.