Logging all bandwidth with router?

okay, how do I go about this?
I need to know how much data is being sent to the internet only, I don't care what's send internally but I need to know the bandwidth I'm using for internet, cause of an action by my ISP with which I'm not too happy (reduced speed because of exceeding datalimit while they advertise they have no friggin' datalimit...
anyway, I need to know so I can cancel my contract without any fees or bitching.

setup:

3 pc's --> router --> cable modem --> internet

router is a Sitecom 202 v3 with the latest firmware, however I highly doubt it supports bandwidth logs etc.
the cable modem is a Motorola SurfBoard 5100, which doesn't support any kind of logging except error logging.