Calculating the so-called street value of drugs is like trying to see into the future with a calculator.
Experienced narcotics cops have a good idea of how drugs are packaged and sold to consumers, and they use that information to guesstimate how much a large shipment of drugs might have been worth down the line if it hadn't been confiscated.
That usually involves dividing kilos and pounds into grams or smaller quantities then multiplying that by the retail value of each bag. Some cops, figuring that the drugs will be "cut" with other substances, use another multiplier. The numbers get real high, real fast.