Change Product price based on discount set by site owners for different domain - Drupal Commerce
The easiest thing is to hook into rules, as there is a rules event, for when the price for a product is being calculated: commerce_product_calculate_sell_price. This event is designed to hook into, in order to change price for discounts and the like.

You can create your own custom action to provide the discount, or try to use the Rules interface to setup the logic there.