I would respond with a smile and a price of $1,000,000. Then when that person stops laughing, I would ask what they would pay for a handmade quilt of that size, and when they tell me that price, I would start laughing. And politely decline.
But to be serious, let's look at the process: discussing with them the colors, fabrics, pattern-lots of cutting?, notions, shopping, designing, redesigning, preliminary approval, cutting, arranging, sewing, errors-another trip to the store?, then quilting it yourself or subcontracting it out. And you have to fit your life in there as well.
I would say if the pattern was easy, min maybe $500. If complicated, $850+.