I have been using vintage sheets for the backs of my quilts lately. I don't care so much for the newer sheets. They are either cheaply made or so thickly woven that the needle makes a "puck" sound when it goes through. But the older sheets, as long as they are in good shape, are so soft and they last forever. I have been able to find a lot of cute ones for only $4 or $5 at Goodwill.
I have used newer flannel sheets also and they work really well (so soft!), but once again, the cheap ones are no good so unless you get them on clearance or at a thrift store, you really don't save much.
The other thing I do is look at the lqs in the sale room- here it's 40% off- and often I can get enough for a back. That way I know it's not going to unravel or tear easily. Cheaply made fabric is so prevalent now. GRR!! If a quilt is going to be a "special" one, with a lot of time or money in it, then I will use one of the fabrics that I used on the front, cost be darned!
I would guess I spend about $40 on a back, unless I find a sheet that works at Goodwill, then it's only $4 or $5. (Been doing more of that lately!)