The Globe and Mail had an interesting article on this. A lot of the points made sense - e.g. economies of scale, increased marketplace allowing for increasing competition, but in terms of labour costs and the overhead of additional regulation I didn't realize quite how high some of the gaps were:

Last year, the Canadian Auto Workers confirmed that employees at Ford of Canada's plants made, on average, $16 (U.S.) per hour more than Ford workers in the United States.

That's not an average of $16/hour, but $16/hour MORE. At a 40 hour work week with two weeks of vacation per year, that would mean that on average workers in that sector in Canada made about $30,000/year more per year than their American counterparts.

The question remaining in my mind: why would Ford have ANY plants in Canada? Is there something that this figure isn't capturing? Is the extra pay per employee coming from government subsidies intended to attract business?