Look at the Helm Control Panel (www.webhostautomation.com) as Helm has built in Billing System and Diskspace/Bandwidth management. You can set up Notifications to clients for over usage and have them purcahse Extras for additional space/transfer.
If the built-in billing system isn't enough for what you want then you can also use ModernBill and AWBS with it.
The pricing is all changing on the 1st December so it's worth buying a license by tomorrow at the old rate for a Control Server, if you decide to go with it. At present the RRP for an unlimited Control is $499, on 1st Dec it will be $899 (but remote licenses will be $299 rather than $499 for multi-server hosts).