If people had to pay the true cost of their decisions up-front, we'd make a lot of different decisions.

That said, I got quite into this stuff a few years back, and determining "true" cost can be harder than it sounds. Externalities, positive or negative, have to be measured against a baseline, and deciding on where that sits is subject to opinion and bias.

You don't need to get it perfect though. The right incentives get you most of the way. Perfect is the enemy of good.