If we pump it back into the oil fields (and stop using oil!) then it is essentially a carbon credit. The cost of doing that would be an excellent basis for the cost of a carbon credit.
We could also just bury the plastic, like we do now. Most plastic cannot be decomposed by bacteria, so most of that carbon will be sequestered from the atmosphere.
It would be a percentage of what was taken out though, given it takes energy to convert plastic back to oil; it would likely get mixed with other oil products instead so that the companies can cash in on government subsidies for using "green" technologies.
