I have made hundreds of quilts with mattress pads, blankets, flannel sheets, fleece, etc. instead of batting. With limited income, batting is just too expensive, as I give the quilts away. Also, I tie them to save time,and for easier care for recipients, and that does not work with batting unless you tie very closely.
If you are making poly knit quilts, you do not need anything inside or they weigh a TON!
We use about 20 old electric blankets per year--it takes about 10 minutes to get all the wires out ,and I just zigzag the holes made to take out the thermostats.....they are usually about $1-3 each.