I have a plan that starts at 3Gig and then gets more expensive when I go over that and up to 5Gig. With regular browsing, my wife playing games on Facebook and my kids playing their games, we come close to 3Gig but have not gone over that yet.
One thing I did do to cut down the usage was installing Add Blocker Plus into Firefox. It has cut my usage quite a bit. You would be amazed at how big some of these advertisements are. Yahoo likes using videos for advertising and they are quite big by themselves.
Rogers does have a calculator for figuring out usage. It gives a bit of an idea of how much you would use. I did find it was a little low when I calculated so be aware of that. My use is a little more.