It's easy to hit 100gb per month, I've known people to do 300mb per month. The 50gb cap is becoming common. Try checking out the other ISPs in the area, more and more companies are reselling shaw/telus so you can avoid the cap that way. Start cutting out serial content, like those TV shows, pick the bad ones and don't watch them anymore.
I believe NetMonitor could tally up bandwidth, as could NetBarrier.....you can use IPFW as well. NetBarrier will keep the number until you reset the counter, not just restart the computer (this is information going back about 4 years, sorry if it's incorrect). It's surprising that your ISP doesn't offer a counter, try phoning them up and telling them to enable it, since you can't monitor your bandwidth usage for them without something to monitor it with.