i would figure the cost of materials (new cotton backing- batting) then figure out how much time it took.
i charge $20 an hour just for sewing.
add what you have into it...then adjust to make the cost fair to you and to her-
some people base pricing on (triple the cost of materials) some charge straight time- some use a combination of both.
this really should have been figured out before you ever touched the quilt.
when someone brings me a quilt for repair i start by looking it over and discussing what exactly needs to be done; estimate material needs- guestimate a time frame and agree on a price.
i always guestimate a bit on the high side- then the customer is happy when they get it back at a lower price then we agreed on.