Natural Resource Economics is a whole field largely devoted to determining the market value of natural resources (trees, rivers, clean air, all sorts of stuff).

Natural Resource Economics is a whole field largely devoted to determining the market value of natural resources (trees, rivers, clean air, all sorts of stuff). It's messy work and difficult for reasons that others have pointed out. But it's certainly a real thing, and incredibly useful for policy-making, where being able to speak in dollar-amounts is crucial.