If I have a customer renting two servers from me, and I collect bandwidth data at the switch port, and he does large transfers between the two boxes that travels through my switch, how do I avoid charging him for that bandwidth? Make him install his own switch? That's the only way I can think to do it. I can limit other people from flooding others on the switch with vlans, but within a vlan, it really isn't fair to bill for traffic that flows between ports, is it?
You could always set up monitoring where his VLAN egresses your network.. or if your network isn't structured properly to allow that, use something like ipfm on a host mirroring your uplinks with the customer's subnet(s).