The marginal damage costs of carbon dioxide emissions are unlikely to exceed $50/tC, and probably much smaller

See The marginal damage costs of carbon dioxide emissions: an assessment of the uncertainties by Richard S.J. Tol. He is at Centre for Marine and Climate Research, Hamburg University, Bundesstrasse 55, 20146 Hamburg, Germany. This is the abstract of his study:

"One hundred and three estimates of the marginal damage costs of carbon dioxide emissions were gathered from 28 published studies and combined to form a probability density function. The uncertainty is strongly right-skewed. If all studies are combined, the mode is $2/tC, the median $14/tC, the mean $93/tC, and the 95 percentile $350/tC. Studies with a lower discount rate have higher estimates and much greater uncertainties. Similarly, studies that use equity weighing, have higher estimates and larger uncertainties. Interestingly, studies that are peer-reviewed have lower estimates and smaller uncertainties. Using standard assumptions about discounting and aggregation, the marginal damage costs of carbon dioxide emissions are unlikely to exceed $50/tC, and probably much smaller."