Simple function question

Ok, here is the short version... I have a small home network. Very basic setup. 3 wireless devices in "my" environment going through a Linksys E1500. The modem is going into said Linksys. I share internet with a couple of people upstairs. I have run a CAT5 cable upstairs to them. They have a DLink router and 3 or 4 wireless devices in their environment. (Only accessing the internet from their Dlink router) All I want to do is be able to monitor the amount of the downloads in gigs going thru the ethernet cable to them upstairs. I have a download cap of 65 gigs a month before my ISP starts charging extra. Every month we are blowing through the cap and I know it's not me. Is there a way to use Wireshark to simply monitor the amount of traffic going thru my Linksys router? (Wifi vs ethernet port) Not interested in piles of information about what they are downloading or intruding on their privacy... Just want the raw upload/downloads in gigs for a set period of time. (monthly would be preferred)