Hot lingerie! $50 will buy a gorgeous bra and matching pair of panties, in a nice gift box. (And Victoria's Secret has their Semi-Annual Sale going on right now, hint, hint!).
Even if she's a tomboy she might like you to turn her into a supermodel...
Other than that: Make her favorite things -- all the things she secretly dreams about, the house with the picket fence, the climb to the top of Mount Everest, the Skydive, the Porsche, or whatever it might be -- out of that clay that hardens in the oven, in lots of strong colors. I always used to say to my ex-bf "I want an elephant for my birthday!", "I want a humongous ornamental plastic donut to put on top of our roof for my birthday!", and all kinds of silly things, and he remembered ALL of those things and made each and every one for me out of colored clay! And it was the best birthday gift I ever got, while it probably cost him <$10. This was 10+ years ago and I still move those clay things with me whenever I move to a new place, 3 boyfriends later and all.
So be creative, and give her what would make her the most happy!