Its a great utility to do exactly this.
iptraf -L /some/log/file.log -i all
You can view the logfile to get the statistics you need. In particular which ip's send the most traffic.
Theres a lot of nifty rules you can setup in iptables to manage connections like this, for example you can add rules to quota the amount of bandwidth provided per subnet or per IP.
If you want to go a few steps further you can setup traffic control and deliberately slow down connections from IP's on your network that hit their quota.
I.E quota = 1GB, if you go higher your packets are classified in such a way that you get a 128Kbit connection for the remainder of the day.