Well, it's interesting because, if you go beyond about 600 miles, it becomes much more expensive because it's very difficult to transmit AC current over long distances or, to put it in technical terms, more than a quarter of a wavelength. B.C. Hydro chose to add series capacitors in lines everywhere. Hydro-Québec chose to isolate themselves completely from the U.S. and connect only with DC.
My suspicion is that, if you were to look at the cost-benefit ratio, you would be better off focusing on renewables in the south and interties with Manitoba rather than a long, long transmission line to the north.