I try to peg it to bottles of wine. That's a generally well-respected gift, and will run you about 15-20 bucks max (if you are getting reasonably nice, but not over-the top fancy wine). So somewhere between 10 and 30 bucks. But really, what will be more appreciated is a truly heartfelt card outlining all the ways they have been wonderful mentors, how much their guidance has meant to you, how much you appreciate the time and effort they put into helping you succeed as a scholar, etc. And regular, unsolicited emails to the same regard. One of my mentors mentioned that she had included one of these emails in her tenure file as evidence of her mentoring, which was touching, to know I had helped her too.