Canadian drug prices are low becuase the government made a deal: in return for a longer patent protection time period, the federal government reviews and sets the prices for prescription drugs.
It's a trade off we made, to help encourage pharmaceutical research in Canada.
Using our low prices to put sand in the eye of the big US drug companies by selling the drugs across the border is counterproductive.
Not only that, but it's technically illegal for a doctor to prescribe drugs to a patient she/he has not seen.