Externalities should be baked into prices to the extent it is practical and desirable. Practical: how easy is it to have a rough calculation of the externality

Externalities should be baked into prices to the extent it is practical and desirable. Practical: how easy is it to have a rough calculation of the externality's cost, and include that in the price? Desirable: how does pricing in the externality help achieve some socially desirable objective?