I don't think we know how prices would change in the long run? It could turn out is that increased volume of sales gives Canada more leverage with its suppliers as a larger buyer.
I think it's much more likely that Pharma manufacturers would cut supply to Canada and pressure the government to price match what US end users would end up paying - it's hard to tell but... I don't have a very optimistic view of the morals of drug manufacturers and this would be a highly convenient crow bar.