1. If they never told you about the overage charges and what they cost, you have cause for a civil complaint. Consult a lawyer on this.
2. If it was buried in your agreement but you never took the time to read or understand it, you don't have any recourse.
3. How does the cost of the card compare with an option like Hughesnet? With overages, does Hughesnet compare favorably?
4. Typing "How to monitor your bandwidth usage" into Google resulted in a lots of hits for programs that let you monitor this and set quotas to keep you from going over. You should be able to find a software solution.
5. Asking for advice on how to get free bandwidth on an EVDO card violates the terms of service for this board and could result in banning.