So many things could be factored into charging for this project. Charging for time is anyone's guess but materials is another topic. I think you would be within reason to start with the cost of replacing the fabric you used for the back, Even though it came from your stash, you paid for it. She has supplied the front and you used scraps for the batting. Unless you had another project of your own planned for the batting, you may be glad to have used that! Batting pieces seem to accumulate faster than fabric scraps for me anyhow!
Rag quilt. I assume you are talking about the one with the "fringe" between blocks. Yes, a lot of work and hard on the hands. I'd suggest starting on doing the clipping on that, showing her how it's done and ask her to finish doing the clipping.This would make the work a part of her own and she might appreciate your time/talent more.
A rag quilt from those soft blankets, ohhhh, I love that idea.