My daughter's dance helper (for 4 years) graduated last year. We gave her $50 cash. Money was super tight for us also, so I cut back on our groceries that week (more hamburger, less chicken) and we gave it to her. Her mom told us it was the most generous gift from the dance girls. I was shocked!
A few people didn't bring anything at all.
So honestly, give what you can and it will be appreciated. When I haven't had money, we've made gifts. How close is she with your daughter? Can you do a framed picture of them with a mat and have your daughter write down different memories on the mat? Or draw pictures?
For my sister one year I found a babysitter poem (I think out of a Chicken Soup for the Soul book) and wrote it on a piece of paper that had my kids hand prints on it. I let them color the paper and we framed it in a cheap $5 frame - all I had at the time. She still hangs it in her house and she stopped watching my kids 3 years ago!
When I was a nanny and went away to college, the family I worked for was extremely generous in the gifts they gave me. My most favorite one was a large copy of the Dr. Seuss book "Oh The Places You Will Go" and the boys both signed the inside cover. They are now 15 and 12 and I still have that....the money is gone and the other items I don't even remember exactly what they were - but I've got the book still!