Does your school have a site license for SmartBoard or ActivBoard software?
Do you already have a board but would like the ability to move around the room and interact?
I purchased a graphire tablet. It is made by Wacom (the same people who make the Bamboo). With the Board software I can have the functionality of a full Board with only a projector and a computer. A graphire is a 6 x 8 bluetooth tablet that works with Windows or Mac. If you don’t have bluetooth on your computer, then you can purchase a USB adapter for less than $20.
I was able to find the tablet online for around $200. It is pressure sensitive and allows you to place a picture in the “frame”. It is meant for tracing a photo. I printed a 6×8 of my desktop to give it the feel of a real screen.
If wireless is not important to you then you could purchase any of the Wacom tablets. Some of them even include touch features. There are currently models priced at less than $75.