If you order tires from US site, the tax you pay will be GST&PST + duty with processing fee if appliable.
If you don't want to buy OEM tire then don't go to Costco, Canadian Tire, Wal-Mart and so on.
For the best quality/price ratio, go to the tire store/dealership for the original.
Check the following link http://diyfromazurewave.blogspot.com/2006/04/on-line-shopping-reviews-for-tires.html and have some comparison you will find some good deal yourself there.
it is acctually ((price+handle+shipping)*(1+duty rate))*(1+GST+PST)
and probably the price difference is more prominent if you buy expensive tires.
tire prices at dealer are not bad when they have deals. also works out when you are not sure what kind of tires you need for your car.
make sure you know/understand the specs. of your tire when you order. especially the offset.(not the profile ratio)