I've been sewing most of my life and often fix things for people. I think if that was her reply, I would opt for a hand made item for her (most people that do some form of craft or art really appreciate the work that went into the item) or a gift card that she would like. I think sometimes its hard to charge people when you don't do things like this for a living. I sell some sewn items and know what to charge for them, but when people ask for alterations or repairs I have to think about the time and materials involved.
I looked at JoAnn's website and a zipper that length was 4.99. I'm not sure how long it would take her to put it in.