My normal tip to my hairdresser is 20%. I would add an extra $5 or $10 for Christmas if I were you.
I just tipped mine $10 yesterday.... on a $20 haircut.
Sounds like a big tip, but I only get my hair cut a couple times a year. So far, she's the only one who can figure out how to get my straight, fine hair to do anything. She's also currently out of work, and I know what that's like, so I paid it forward a little since I was recently blessed with a full-time position in my field.
I always pay 20 percent or more. A lot of one's image, relates to how their hair looks. Attitude is important and how one feels about how they look can affect self-esteem, in some manner.