Since the newest Airport Extreme routers don't support SNMP, how can I monitor bandwidth usage on my network? I am currently using about 600gb a month and Comcast is about to start charging me for anything over 300gb. I need to find out which devices are using the most bandwidth so I can get it under control. 

The newest airport extremes (the tower version that supports 802.11ac) do not support SNMP. Is there any other way to monitor bandwidth usage by device on my network?
PFSense, DD-WRT/Tomato, Peplink, or some other router in front of the Apple AirPort Extreme will likely be the best solution.  Don't think there's an easier way unless you're running on a single machine for your whole network (not likely).  All the routers I mentioned support realtime traffic monitoring.  NetGear Nighthawks have this capability too.