FTP is going through
Just want to make a note that even if no firewall rules are entered on my LAN side, the FTP is going through, probably because of the FTP Helper. You can block it by explicitly creating a rule at the end of your rules that will block everything. Probably disabling FTP Proxy will also correct this issue.
It cost me 5000$ because of a laptop upgrading his virus defs using FTP over my satellite connection.
It is always a good security practice to create explicit allow rules for services under any internal zone, and change the default Allow All to Deny all. Never use the allow all. :)
It's also a good practice to know what runs on your machines, when and how as well.
Sorry to hear about your Internet bill, however.
In case you have to pay for traffic I would frequently check the rrd graphs. It should be an indicator for unusual high traffic. If you see something there it's time to monitor your traffic, either by viewing pftop from the shell or checking diagnostics>states. You should find the sucker pretty quick then.