We are always our own worst critics. Your work is definitely good enough to charge the standard going rate. You did custom quilting on this and as PP stated that usually starts at .025 to .03 per square inch. The quilt is 2,688 square inches so if you take the lowest price of that you are already at $67.20 just for the quilting. Then the binding is usually charged by the linear inch and you have 212 L/I of binding. I believe the going rate for that is around .10 but that may be for an attached binding not bringing back to front. Still I feel $20 is more than fair.
I would present her with a bill for $87.20 if she balks at that you need to be able to come back and tell her how many hours you spent doing the job. I am sure if you worked it out you would be making MUCH less than minimum wage.
I agree that a price should have been agreed upon before hand but you shouldn't have to be the only one doing the learning IMHO. The person who asked you to quilt should also learn a valuable lesson from this.
You did a very nice job.