Unfortunately, in an e-commerce setting, you can't always wait until you have a total to show what the products will cost. You need to show the customer a price for one unit of a specific product (sometimes even with the discount broken out) in their own currency and then the numbers all need to add up to the correct total once they put several items into their cart. You end up getting penny off errors here and there that even if they are in favor of the customer, look very unprofessional.
Our solution was the opposite of your suggestion. We determine the converted price of one unit of a specific item (including discounts and exchange conversions) before adding up the total because then we can count on it being consistent across the site and through the checkout process.