Give `tc` a try, it's a bandwidth shaping utility that could possibly prevent your overuse problems before they start.
Other than that iptables will give you the number of total bytes sent through various rules, so you can query that cleverly and determine how much band you're using.
This is a really generic example from my system (where all gatewayed IPs are agglutinated, but they don't have to be)
Code:
root@gateway:~# iptables -t nat -L -v
Chain PREROUTING (policy ACCEPT 34224 packets, 11M bytes)
pkts bytes target prot opt in out source destination
Chain POSTROUTING (policy ACCEPT 5298 packets, 274K bytes)
pkts bytes target prot opt in out source destination
10167 542K MASQUERADE all -- any eth0 anywhere anywhere
Chain OUTPUT (policy ACCEPT 15386 packets, 810K bytes)
pkts bytes target prot opt in out source destination
