How are people dealing with Web Filter/Cache Engine/Proxies for Billing?

We have the netflow analyzer and the billing module. We currently have been planning to bill on our aggregate but are unable to as it has proxy server IP addresses hiding multiple customers on the back end. We also have netflow on the port that connects the proxy server people are using and being hidden behind. We'd like take the aggregate and negate the proxy's IP address and then add in the IP traffic from the interface that connects the proxy for billing all traffic and the traffic going to the proxy but removing the proxy's IP. We'd like an aggregate view of the combined traffic to the proxy with all the traffic to our carrier.

Anyone else dealing with something similar and how? We're the looking for the best way of doing this in the Netflow Analyzer to make the billing module provide us with what we need to be fair to our customers.