Here's a study that puts the numer of Canadians using the US healthcare system at about 0.61%: http://content.healthaffairs.org/content/21/3/19.full.html
As for that study, I'm a little concerned that they only looked at a handful of states, and this little line at the bottom:
"The authors acknowledge financial support from the Canadian Institutes of Health Research (formerly the Medical Research Council of Canada) for this research."
My guess: both your source and mine are biased and flawed, and the truth lies somewhere in the middle.