I just chalked it up to coincidence. Sure, shooting in Canada gets you tax credits, but to go for the cheapest actor when you're shooting in LA doesn't make a lot of sense. That second-rate actor you hire might mean you're not getting an actor who can take off like a rocket in the role, and lock in a loyal audience that will ensure the series' success.
Basing casting decisions just on cost strikes me as very penny-wise and pound-foolish. You need go no further than Keifer Sutherland to see that. Regardless of what he cost to hire, his hiring was the single biggest element in making the series a success. Bad casting in that role could have sunk the ship.
If this isn't how it works, why do actors in general get paid so much? Why don't we see a continual parade of new actors in movies and TV? There's a continual supply of hopeful wannabees, so why would anyone ever pay an actor more than the most desperate wannabee? The supply and demand pressures would drive all actors' bargaining power down to minimum wage levels and there would be no such thing as stars.
