As the developer of a REST centric ecommerce platform[1], I agree. The price of an order item is specific to that order and does not necessarily match the product's base price. It may be calculated using special account pricing rules, tiered by quantity, or discounted by a coupon. In such a system I have found many occurrences where data has to be de-normalized for this reason. Although the the new state is derived, it may otherwise differ from the source. Not just a function of value and time.
1) http://getfwd.com