First, I'd like to point out that TF2 is online. Online games don't have a
lot of data transfer, but it is a constant bandwidth drain, and you sound like you're pretty limited. Single-player TF2 against bots is possible but horribly unsatisfying.
* Upgrade your internet connection! Paying $100 in overage fees is obviously ridiculous, especially when you could probably upgrade your monthly allotment for a fraction of that cost. (Unless something else precludes an upgrade...?)
* If you can download it via an internet cafe, consider buying a large flash drive to transport it home. A 30-second look at Future Shop shows a 16-GB flash drive for $10-15, so portable memory isn't terribly expensive.
* Purchase a cheap netbook. $100 would get a passable machine that's 3-4 years old, which you could then use on a different network to download the game.
* Ask a friend! Surely someone at school has a decent connection. Heck, they might already have TF2 installed! If so, you can copy the files over via flash drive.
* If you do this, start the Steam download, then stop it a minute later. This should create the correct directory structure, so you know where to copy the files.
* Once you've copied them over, start the Steam download again. Steam should spot the files, validate them, and report a success.
Also:
* Buying the Orange Box almost certainly wouldn't help. I'm not sure whether TF2 has any on-disk data (some Steam games are simply a download code), but even if the game
was on the disk, you'd have gigs worth of patches to download.
* Check your latency with
Pingtest. If you're limited to only 5 GB/month, I wouldn't trust that Verizon has given you a decent latency, either. This goes double if there's a circumstance (like living in a rural area) that prevents you upgrading your monthly bandwidth.