Wouldn't airodump, EtherApe, or any other application local to the client simply monitor ALL of the bandwidth on the NIC, regardless of whether it stays within the LAN or goes out through the internet? I know for my machines here, 95% or more of their traffic is to other machines on the local network, and would have no effect on the speed of the internet, which is what the OP is interested in. I would think you would need to monitor this on the device which is doing the switching and determining whether the data stays local or goes out through the internet in order for the OP to glean anything useful, which would be the router.
Anyway, as for your question Hak5fan, I have an Asus RT-N16 running TomatoUSB 1.28 firmware, and it has the ability to track real-time, daily, weekly, and monthly total bandwidth, as well as bandwidth for individual clients separated my MAC or IP address. To be honest I've never used the individual client bandwidth testing though, so I have no idea how well it works.
Last edited by suicidaleggroll; 01-27-2012 at 02:11 PM.