Well not updating dat files pretty much makes antivirus pointless. I believe by default they look for an update every 4 hours, but it should be an incremental update and really small to be honest.
Now you can manually update here http://www.avast.com/download-update
, looks like about 40MB for the manual update.. Which again -- your normal updates should be incremental so much much smaller.
You could turn off internet update and download manually ever now and then during the month I guess, or could have one update and use it to update your other machines. I don't use that product, so not sure of details of moving updated dat files from one machine to another.
But I find it highly unlikely that updating your dat files is cause of your over cap..
"stopped downloading on demand video"
That would be your biggest hitter if you ask me.. Lets say you had all 4 machine downloading that full 40MB a day for their dat updates.. Which again - they should be incremental and a small FRACTION of that size.. But lets say it was the full 40MB, your taking 4*30 your at 4.8GB in a month.. What 3% of your total?
Do you have anyway of tracking access at your router? For example
If you can not monitor at your router, put something on each device and see what your heavy hitters are, etc. Something like http://www.netlimiter.com/
and FREE monitor version that can show you what applications are download, what your totals are, etc.