In Canada, under the Wireless Code of Conduct mandated by the CRTC (roughly the Canadian equivalent of the FCC) since December 2013, once you go over your plan's data bucket allowance, carriers can only legally charge at most $50 for domestic overage, or $100 for international overage, based on their insane rates. It's usually $0.01/MB - $0.05/MB for domestic overage depending on the plan, and even more outrageous for US and International usage that you wouldn't believe.
After you go after that amount of overage in usage, most carriers will block you from further data usage until you contact them and consent to further overages.
To my knowledge there is no such carrier in Canada that does a more sane approach of throttling your data speed once you exceed your cap, and allow to buy more 'fast data', like it's possible with some prepaid plans with other providers in the world. The only exception is with Roam Mobility, but they operate as an MVNO for carriers in the US to provide cheap roaming for Canadians travelling to the U.S.