Commissioning a friend to sketch?

Hi everyone, just wanted to say this group is fuckin awesome. Anyway, I'm planning out a watercolor tattoo and my best friend suggested that I ask her younger sister who is in art school to sketch/ paint it out for me. Has anyone had experience with doing this instead of asking a tattoo artist at a shop? I assume it's no different but how do we discuss pricing without either of us feeling like we're taking a loss somehow? Thanks for the help and sorry if it's a silly question.