The Social Cost of Carbon: that means, "how much trouble are we setting ourselves up for, for each extra ton of CO2?"
The EPA says it's $30ish dollars per ton, maybe as low as $10, maybe as high as $100. Those numbers are uncertain because most of the trouble happens decades into the future, and to convert from (e.g.) 2050 dollars worth of trouble into 2016 dollars, you need to know the future's inflation and interest rates. Which nobody knows, of course.
So, how much is that per barrel of oil?
Since one barrel of oil from a well creates about 1/3 of a ton of CO2, the Social Cost of Carbon is about $10/barrel (maybe as low as $3, maybe as high as $30). That's NOT a small number, considering that a barrel of oil costs about $40 these days.
If one forced oil companies to consider the social cost of carbon, their profits would be gone.
Oil to CO2 conversion: http://numero57.net/2008/03/20/carbon-dioxide-emissions-per-barrel-of-crude/