IP bandwidth usage collector?

[gashalot () gashalot com]

This might be a little off topic, but I run a fairly large network, and
we sell dedicated servers to clients.  All of the dedicated machines are
connected to our network of Cisco 2924XL-M Catalyst switches.  I would
like to be able to collect bandwidth usage statistics for their ports
that way I can easily bill them for overage, and I don't want to have to
install anything on the machines themselves to remove the threat of
tampering.

Does anyone know of a package that does this?  Or would it just be best
to write some quick Perl hack to pull the bytecounter out of the Cisco's
SNMP information? 

Thanks for any help anyone might be able to provide.

--
Robert Gash
System Administrator, FastURL
Personal Email: gashalot [at] gashalot [dot] com