I don't wonder if I'm good enough because the only person I need to please is me! If you're not certain that your current skill level justifies taking on a commission quilt, show her samples of your completed quilts so she can be the judge.
Beyond that, once she understands the true cost of making a quilt, she may back out of her request and you need not worry further. If she is undeterred by the cost factor, then you have to consider the imposition of her projects on your time and the fact that your projects will fall by the wayside as you become tied up with hers.
Good luck with your decision.