I would say $20 easily
How about making a tag on each saying
FUNDRAISER TO SUPPORT QUILTS FOR KIDS
MIMIMUM DONATION $20 . . . (or an amount you want)
(Value of fabric $______ )
By listing a dollar amount you have into the apron for fabric, the person *may be able* to use the difference as a tax deduction, if you're registered as a charitable organization. I'm not a tax person, but I believe this is how it works.
Depending on your event - how about raffling them off. $1.00 a chance or $5.00 for six chances or $10 for 14. People involved can start selling chances now before the event. Make copies of the pictures to give to people selling the chances.