monitor/log external only traffic

I have an iMac as a home server, connected via ethernet to a home wifi router.
I control it via Screen sharing from macbooks, or my iphone with a VNC app.

Since installing io6 and lion, I have setup icloud, and now suspect this may be the cause of enormous data usage on my home internet connection (NZ has limited data caps due to our limited cross ocean cabling).

My question is this:
Is there a way to monitor/log my iMac's network activity, but only the data to and from the WAN. I don't want to count the movie I stream over the network to a laptop, or my VNC data usage.

I have used surplus meter in the past, but for some reason PPP monitoring does not count anything and obviously monitoring the ethernet port will show ANY data usage.

Thanks for any suggestions.