transfer limits

Hi, I am pretty new to this board, so I dont know what was posted in the past. My question is where can I get a script or program that will keep track of the data transfered in a certain amount of time on a per-user basis?
Currently I am interested in starting up a website hosting company and I have been looking at other sites rates, and I keep seeing "up to 20G of data transfer per month" and so on, I get all that, but I would like to know how they do that. I used to administer a site that I paid a hosting compay for, and they kept track of it everytime you logged, in "234567 of 23456789 bytes transfered this month"
Any suggestions or comments are welcome, please e-mail me.

-Chris Breitbach