A quilter is worth every bit as much (likely a whole lot more) than any mechanic (sorry, we call them automotive service technicians now, don't we).
The national rate (straight time) varies (location, foreign/domestic, etc.) between $15 and $22 per hour. That's what the mechanic makes; it costs the employer lots more because of contributions, added taxes, etc.
You are worth all that and more including your time to design, shop for the fabric, etc.
If it's a rush job and you have to go into "overtime," then adjust your hourly rate to time and a half or more appropriately (as mechanics usually charge for emergency repairs), double rate.
Actually, when someone asks what you charge to make a quilt, a good reply might be: "Well, the cost of the material of course, and what does a mechanic charge in these parts ... that ought to be fair."
If they're serious, they should not be put off by that and you will be making a fair hourly rate. Otherwise, you lucked out and didn't end up doing something for nothing.
Hope that gives a bit of perspective. Good luck with your venture.