Monitoring throughput on SMs with MRTG

Can I monitor the throughput (in bytes) for all my SMs by running MRTG against the AP? Or do I need to run it against each of the SMs? Problem with running it against the SMs is that I cannot ping the SMs remotely. The router I have is a Dlink and I can get to the AP through port forwarding – but then how do I get to the SMs?

Bottom line is that I need to know the total throughput (in bytes) used by each customer per month so I can charge them accordingly — their plan includes 1GB of throughput per month and I need to charge for overages.

Gregg Smith