I use Comcast and I am looking for the same thing:
IPFire (old) Forum - Help Monitoring Data Caps
IPFire (old) Forum - Traffic accounting
The closest I came to monitoring each device was using pmacct (and with a ton of help from Erik @ummeegge!)
But I had to post process everything in Excel using pivot table to get the info I wanted. And I could not determine how to make that happen via a script.
IPFire (new) Community - Pmacct - A lightweight passive network monitoring tool