How do you measure them and how do you set a price on their cost? When they are in a gray area, how do you decide if it's truly negative? Should we negatively

How do you measure them and how do you set a price on their cost? When they are in a gray area, how do you decide if it's truly negative? Should we negatively tax something with a positive externality? Who gets to decide, and how can I be that person? (A permit auction does not suffer from these problems)