I have found that saying, "I don't quilt for money. If you really want one, I will show you how to do it." usually solves the problem.
If you do decide to do it, set your price high and use an approximation ("It may cost as much as $300 for my time and additional materials.") And get a deposit of at least 1/2 of the price you quote. Also, make clear that you won't deliver the finished quilt until you are paid in full.