My point was that the added cost should be reflected in the collection of waste, not the point of sale, because there aren't any externalities yet, and so there isn't anything to calculate.

As for landfills being cheap, that isn't a bad thing. That means the externalities at those sites are low. I live near an old one that was turned into a 50 acre prairie restoration space with about 2 miles of walking paths and full of wildlife. As others pointed out, burying carbon does sequester it. Methane capture merely prevents it from getting back out when it is nutrient and water dense (aka food waste).