Right now the USD exchange rate is about 1.36% so my current .com renewal in Canada is...
$15.99 x 1.36% = $11.73 USD but most credit card will add another 2.5% which brings it to $12.02 USD per .com domain.
So you have to set the $12 as your baseline for the cost of a .com in USD should you buy in Canada
So at $5 and $7 there is a significant savings
You also have to check the sales to make sure it is not only on new registrations, check to make sure transfers are included before making any decisions.