How much should I pay my friend to teach me piano?

We’re both sophomores in high school, and she’s pretty brilliant with the piano as far as I’m concerned, as she writes her own music and performs it for classes and such.
We’re close, and I asked her if she would teach me and how much she would like to be paid for lessons.
She said she wouldn’t want to be paid, but I insisted that she should be because I’m a difficult student. :]

So how much do you think would be a good salary for a high schooler who needs some extra cash?

$15 per lesson at least
my piano teacher charges $90 per lesson, so it can get pretty expensive. I teach my friend to play the piano too (; have fun!