what about a "valdine" key card system like some student housing uses? Kansas State University's dorms had this and it was a good solution to the problem. I think it's just a magnetic trigger set to a certain frequency supplied by the card when it's scanned through the reader, which looks a lot like a credit card reader. If you're looking for cheap, check with your local colleges. They might be upgrading from this system to something newer currently.
If that wouldn't work, why not just a simple RFID chip reader connected to a lock? The building I work in right now has that kind of system and the keycard is the size of a credit card. You don't even have to take it out of your wallet to use the thing.