Looking at prepaid SIM card options in Canada it seems that the best choice by far is to get a T-Mobile prepaid plan for $50/month which includes 4GB of data and allows you to use the same tariff in Mexico, US and Canada. The detailed T&C's mention that:
Not for extended international use; you must reside in the U.S. and primary usage must occur on our U.S. network. Device must register on our U.S. network before international use. Service may be terminated or restricted for excessive roaming or misuse.
However it's not clear what "excessive use" means. Is it one month outside the US? 6 months? 1 year? If they let you roam for at least a month it would be perfectly fine as one could easily stash numerous anonymous prepaid SIM cards and swap them out once per month.