Depends on how they are charging. Usually it is the length in inches times the width in inches times the price per square inch.
So if your quilt was 40 X 60 and they were charging $0.02 per square inch it would be:
40 X 60 = 2400 square inches
times .02 = $48.00
If they are charging per square yard it's a different number. There may also be addtional charges or a minimum charge.