Dressmaker help! How much should I charge?

Okay so I'm an 18 year old student still attending high school, and I'm taking a fashion techniques cl. I design and create dresses to one of the professors at my school. I guess you could say I have good sewing skills, thats what my fashion teacher said. But two weeks ago my professor the one that i work with, asked me to fix this coat for her. She needed me to hem it, she had taken it to the cleaners and the threading at the bottom was undone and torn apart. So it was a long coat, and had an opening one each side, making it three parts(if that makes any sense) anyways, the middle part she wanted it to be long, so i just fixed it without cutting any pieces and hand sewn it together, then the side ones i cut about an 2 inches and hand sewn it as well. Then i fixed some other damages, then on hole was becoming undone, and at the top, inside of the coat, the satin textile was tearing, and at the side of the coat it was also tearing so it fixed all of this by hand. So I was thinking of charging her 15, is that a good price? I mean i hemmed pants before and i charged her 3 bucks but she said it was too little so she gave me 5. So do you think this price is reasonable 15