The Globe and Mail‘s Hugh Thompson has an excellent run down on this. The key quote from the article:
Assuming an inflated cost of 10 cents per gigabyte, it means that Bell, Shaw and Rogers are charging consumers between 10 and 50 times what it costs them to deliver data. This on top of their regular monthly Internet pricing! While I agree that heavy users should be prepared to pay more once they have reached their bandwidth caps, a fair price would be much closer to 10 cents per GB than the inflated $1-to-$5-per-gigabyte charge sanctioned by the CRTC.
I highly recommend the article: great analysis and reporting.