Which cost of materials to use?

Dear all, The retail price of a certain product is $15 (anyone can buy it from the supplier for $15). However, the contractor has a special discount: he buys this product for $10, but he bills the client $15). When calculating the percentage of regional materials by cost, do I have to consider the $10 or $15? When I say the contractor bills the client $15, this means that the BOQ states $15. Many thanks, The LEED Reference Guide states that "Materials costs include all expenses to deliver the material to the project site."