Hi R.,
If you are good with a computer and have a video camera, our room moms put together a little film with each of the kids saying what they liked best about their teacher. One of the moms set it to music and put a little graphics into it, and it was a really great teacher appreciation gift. Something like this might also make a nice retirement gift.
My guess is that the teacher would rather have a gift that came from all the kids rather than a purchased gift from the store. It would be more economical for you, and it will be something that your teacher would be able to enjoy for many years to come, especially since it is her last year.
If you would still like to purchase a gift for her, perhaps you could ask parents to pick up gift cards of $5.00 value to a few fast food places or ice cream stores. With those gift cards, you can make a gift card tree (a little tree with some cute, miniature clothes pins shouldn't cost more than maybe $10 or $15).
Good luck with the gift. I'm sure that anything you make or give to your child's teacher will be most appreciated.
Take care,
L.