Bandwidth monitoring

[Adam]

My ISP is placing strict restrictions on how much I can transfer each
month, with high penalties for exceeding their limits. However, they
don't provide any way for their customer's to check to see how much
they've transferred, so we end up transferring far less than what we are
allowed, just to make sure we avoid paying the fines for going over the
limit.

So, what I need to do is find a way to monitor my total bandwidth
through my external NIC. My gateway is running FreeBSD 4.8 with
ipf+ipnat. 

I *don't* need anything fancy. All I need is to be able to check at any
time how much I've transferred since the first of the month. What's the
easiest way to set up something like this? I know there are fancy
solutions with graphs with usage stats and such, but that's not what I'm
after.

Thanks for your advice,

-- 
Adam <blueeskimo at gmx.net>