As a teacher I can tell you that it is not the dollar amount that counts. My favorite things I have recieved are: Post It notes, pens, markers, unusual stampers, note pads especially those with my name on them, educational posters,desk calendars, picture frames, fruit, snack bars, candles are okay but the fire marshall won't let you keep them at school. Almost anything that can be used in the class room...most you can find at the local Dollar Store so you don't have to spend a ton. Another great item is anything you and your child (or the child) have made...Over the years I recieved ornaments that were homemade . I have a tree at home that I adorn with just such items and its a great way to remember the kids.
My favorite was a riendeer hear made of abandoned puzzle peices
Hope this helped.