It's an oft-repeated statement that "everything" in Canada is overpriced, and while it is true that some goods are less expensive in the US than in Canada due to a number of factors, from economies of scale due to a larger and more evenly distributed population and factors such as supply issues (at least for milk), it is also worth keeping in mind that prices vary considerably within different regions in the US as well.
For example, I had been on a business trip to New Jersey and Pennsylvania a few years ago and I had the opportunity to stop by a few malls along the way, and what struck me was just how expensive many items were in those stores. In fact in many of the clothing stores, the items I saw there were more expensive than the same items available in Canadian stores, not to mention the same items in stores in the suburban areas outside of Detroit (where I have relatives). I had also been to a business trip to Raleigh, North Carolina and when I also didn't notice prices at stores or restaurants to be too far from prices I pay here in Toronto. So it strikes me that the big price differences are more of a feature of border cities/towns.