When I was looking at the Traffic Monitor I noticed that my Chromecast was in my top 5 clients. This chart is only about 5 days into my using the Asus RT-AC68R.
I found this thread on reddit. In that thread there's a link to a earlier thread.
Bottom line is that a chromecast will use 8-20 MB per hour when turned on and idle no matter what. Disabling or changing backdrop settings does not disable the background images, it just customizes them. When backdrop is disabled, new images still appear at the same rate, just with a message about enabling backdrop in the lower right instead of information about the image.
The workaround I found was to power the chromecast from the TV's USB port instead of using the wall adapter, so it's only on when the TV is on. This isn't perfect, because the chromecast is still on when watching regular TV. Additionally, the chromecast takes time to start up. Unfortunately, one of my TV's doesn't have a USB port so I can't do anything about that set.My old TV didn't have a powered USB port. When I migrated to the new TV I just moved the Chromecast along with the wall adapter USB power. My new TV does have a powered USB port so I moved the Chromecast's USB cable to that and the problem is