I suppose it would depend on if it were a friend or just a person wanting you to sew for them (customer).
Keep in mind that your time/talent is worth something and your pattern and thread cost you too. Then there is your electricity and anything they forgot but is a part of the construction basics. Like maybe magnetic closures, fleece or batting, interfacing, d rings, handles, whatever.
I am thinking that the forty dollars she originally charged was broken down into all these supplies and her time. I would subtract for anything she bought/brought and charge the remainder of the balance for my work and supplies. After all she was happy with the price she was asking for the one that everything had to be bought and shopped for.