I dispute this. Given free trade, why should something that sells at Home Depot in Toronto cost more than something that sells at the Home Depot in Buffalo? Why can I drive an hour south of Ottawa, cross the border, and buy something from the stores in Ogdensburg, - which is a burg - and it's cheaper than in Ottawa?
When the dollar was at par, people would cross the border and prices in Canada did drop.
Taxation, energy and wages have something to do with it too.