I wouldn't do it. If I'm spending my time doing something for somebody else I don't get to do my own stuff. Plus people rarely want to pay what a quilt is actually worth in time and materials. They're still of the mindset that "homemade is cheaper than store bought." Which was true before every thing came from foreign countries. When you can buy a "bed in a bag" for less than $100 and even the "fancy" store bought quilts are less than $100 people generally aren't willing to pay $1200 and up for a quilt. Even a nicely done lap size quilt will appraise at that much.
Just smile and thank her for thinking of you and tell her your time is more valuable to you than what she's willing to pay for it.