Around £10-£15 ideally.
It's for DS1's girlfriend, she's 16, we see her a lot and she's lovely so we want to get her a small gift. I'd usually go straight for bath stuff but she has very sensitive skin so smellies/toiletries are out. A gift card seems a bit impersonal but I'm not really sure what else to get...no teenage girls in the family so it's a bit of an unknown area for us!