Prepaid service in Canada sucks ass.
Services like Wind and Mobilicity are dirt cheap, offering unlimited voice and data, but stop working as soon as you leave your major urban areas. They also use AWS frequencies, so unless you have a phone that works on 1700 HSPA you're SOL. If you are going to Canada first, look at Wind because they also have an offer where you can get unlimited US Roaming on your account for $15, this would save the need to pick up a second SIM while in the US.
The big 3 (Rogers, Telus, Bell) frankly don't give a crap about prepaid service, and charge so much for it that it makes more sense to go onto a contract plan and cancel service the same day you leave the country (no more 30 day notice required thanks to the CRTC and the WCOC. Rogers gives the best compatibility with hardware (GSM 850,1900, a fist full of HSPA and LTE Frequencies as well) where Telus and Bell only support HSPA (850, 1900) or LTE on a handful of frequencies.
If you are just going to be in major urban areas Chat-R Wireless (which is just a Rogers Wireless sub brand) has the best rates, but as soon as you are outside of a major urban area your pay through the nose in domestic roaming charges, and the biggest data plan they offer is 200MB for a month (if this matters to you). I'm not 100% sure, but I don't think Chat-R offers a nano sim, so if you have an iPhone 5/5S/5C or an HTC One M* you're SOL as well,
Expect the SIM to cost you between $10 and $20, as well as your first month service upfront.
Disclosure: Industry pro, I've worked in the Canuck retail telecom industry since '99. I've worked for every provider in the country in one aspect or another, and they all suck, I'm cynical and jaded about it, so take advice with a grain of salt, the size of a Buick.