The difficulty of establishing the value of ‘externalities’, including environmental factors, in monetary units has been addressed in several ways: one is to use shadow prices. A shadow price is a value that reflects the true opportunity cost of a resource or service. The real value of something reflects the most desirable alternative use for it. For example (in the case of industrial production), the opportunity cost of producing an extra unit of manufactured goods is the lost output of childcare, food production or whatever, forgone as a result of transferring resources to manufacturing activities. In consumption, opportunity cost is the amount of one commodity that must be forgone in order to consume more of another (Todaro, 1994).