Relative value. Relative value means, except for purposes of 191.51(b), the value of a product divided by the total value of all products which are necessarily manufactured or produced concurrently in the same operation. Relative value is based on the market value, or other value approved by Customs, of each such product determined as of the time it is first separated in the manufacturing or production process. Market value is generally measured by the selling price, not including any packaging, transportation, or other identifiable costs, which accrue after the product itself is processed. Drawback law requires the apportionment of drawback to each such product based on its relative value at the time of separation.