I'd probably charge around 3 cents per square inch at a minimum, as SID is very time consuming. It would make a difference, though, as to how many pieces are in each block - if it's a fairly simple block with large pieces, it would be easier than a more complex block with 144 pieces! Have you suggested an edge-to-edge/overall pattern to her? That's what I generally suggest when the top is so busy the quilting won't show very much.