I'm with those who name a price to see how fast potential customers will walk away. And if they're serious, I'd take them to select and buy the fabrics and batting, and obtain half the labor and quilting cost up front. And then I'd work non-stop to give them a great quality product in record time.
If you wanted custom window coverings or furniture coverings, you wouldn't expect to have it all done on spec, a significant deposit would be required.
The response to the Walmart comment would be "Oh so you don't really want custom work or a one-of-a-kind quilt, no problem - I'm sure Walmart has exactly what you need".
Okay, I'd only think about saying that last thing, I'd just nod and smile and walk away.