netstat -s

will give you statistics on your network (total packets/bytes transferred, as well as a lot of other info). Might be more than you need, but it might let you extract what you need.

You might also check the cable modem. Many of them include network management features, including packet counts. That would be a more accurate figure anyway since that's what your ISP would measure and wouldn't include any local traffic you have on your internal network (e.g. Mac to Mac traffic, printer data (if you print over a network), etc.)