Answer:
There are several resources that you may wish to review regarding usage based billing. The Yankee Group published a report in 1999, entitled "Usage-Based Billing:
Bandwidth Savings with Minimal Downside"
Other resources include:
Usage-Based Billing and Chargeback: New Functionality Emerges for the Enterprise
Applying financial best practices to ITFirst published on BankersOnline.com 5/20/02