thats a good point, I'd argue given that CO2 concentration in atmosphere is a public bad (as in taking ability to sustain life out of 'common biosphere'), the cost to remove it on a per unit basis represents a good measure of externalized costs of unmitigated pollution (just co2, ignoring other pollutants for the moment).

this is doubly good because it provides a great incentive for the free market to minimize the costs of CO2 sequestration and hopefully will let us hit the knee of the optimization curve asap.