Putting that issue aside, the question then becomes, what is that cost? Any price you put on it is simply made up.
Actually its quite simple. The cost of sequestering the carbon dioxide produced along with the energy. (Which could fluctuate with market/technology). In other words, the extra cost should be the same cost to clean up the extra pollution produced. Otherwise the polluter is being subsidized by a price paid by others.
Say you compare bio-diesel with regular diesel, bio-diesel would be expected to cost more in dollars because it involved removing CO2 from the atomosphere. To be comparable with regular diesel you would have to include the cost of removing the CO2. That would be an exact and non-arbitrary price, since the CO2 pollution causes the damage.
We just don't like where these numbers lead and no one wants to penalize their country by acting responsible while everyone else just pollutes the world to their economic advantage.