Wow, surprised nobody has responded to you yet.
What you are looking for is usage based billing. Zuora has a solid hold in this space and from what I understand, implementation can be somewhat simple. Beyond that, remember what you are looking for is usage based billing. This usually is some resource that is consumed over time such as bandwidth, disk space, etc. However, you can utilize usage based billing for CPM/events.
If you want to track everything internally and simply use a billing/payment platform, I would recommend Braintree. Instant integration with PayPal and credit card payments with almost NO implementation hassles. They'll keep track of transactions, vault your customers and give you peace of mind with your PCI compliance.