I was in Canada last summer for a couple of weeks, and Verizon offered me fairly reasonable data (and voice, fwiw) roaming rates. They even automatically canceled the plan on the day I was scheduled to return to the US without me having to make another call to customer service. Most Verizon reps have actually heard of Canada. It's not like you're traveling across the ocean or anything.
By “fairly reasonable,” of course, I still mean too much money, but still less than signing up for an entirely new prepaid plan with a Canadian carrier. Call them up, see how much money they want, and then make your comparisons from there.