When you tell here he going price for fabric, thread, etc., I think she'll say 'Nevermind'. Also explain approx how many hours it takes to make one and that you don't have one 'laying' around. Every one that you make has a specific need in mind and the list is long and growing by leaps and bounds.
I find that once someone gets a little 'hint' of what a quilt costs both in materials needed and time, they have a different outlook on having you 'whip' one up for them or having one 'laying' around.
The idea about buying a charm pack and explaining to her how to make a rag quilt is also a good idea.