IP bandwidth usage collector?

You're pretty darn close actually. For the list, I'll be writing a
little script that will pull these values from SNMP and store them in a
SQL database, so the admin script as well as the user can check their
bandwidth charts (I do use MRTG, but I like being able to look at one
screen and see the exact amount of traffic that my customers have
transferred to the net as well as what they owe us (if anything). I'll
hopefully start work on it tomorrow afternoon and might have something
finished by the end of the week.

  Why not just tally their values out of the MRTG logfiles?
If you're already running it and all..