Then the cost in externalities would do the same. That's the whole point of pricing in externalities: if the paper packaging becomes cheaper after you do it, th

Then the cost in externalities would do the same. That's the whole point of pricing in externalities: if the paper packaging becomes cheaper after you do it, that's because the plastic packaging actually is worse for the would-be dollar-saving population in the long run. Doing the pricing accurately relies on a bunch of projections we don't have, of course, but we can certainly do better than calling it all zero.