Recording and displaying total internet traffic.

Now I have 3G radio broadband, this is fast but its also expensive. I wish to be able to show the total amount of data thats travelled both ways on a particular interface, eg eth1 or ppp0. The idea being I can easily see how much data/dollars I have used to allow me to manage my internet usage better. It would not be chronically difficult to write a script to be run with poff that displayed total traffic as displayed by ifconfig. I wonder if there already is a way around this. Lindsay