You have the following options as far as i can see.
1) Provided your router supports it, you can use open source firmware like DD-WRT or Tomato to monitor bandwith usage at the router level. You'll need to check whether whether your router is compatible before doing that. See this thread. Monitor Your Monthly Bandwidth with Your Router - Networking - Lifehacker
2) You can use freeware apps like Netlimiter (just google) to monitor traffic on individual PCs but to do that for your entire "household", you'll need to install the software on each of the PCs and then add up everybody's score to get total usage at the end of the month.
3) You should be able to login into your ISP's website and check your download details from time to time. This will give the total usage of the common connection.