Answer: $0.35/gallon to $1.90/gallon more than it currently does.

Calculation:
(0.00907 tons CO2/gallon) * ($39/ton) = $0.35/gallon
(0.00907 tons CO2/gallon) * ($209/ton) = $1.90/gallon

Even when accounting for the social cost of carbon, the Prius doesn’t make economic sense versus the Honda Civic for my driving habits.

Gallon of gas produces: 0.00907 metric tons of CO2 (20 pounds)
Social cost of CO2 (low): $39 / metric ton (US Govt estimate)
Social cost of CO2 (high): $209/ metric ton (mean + 2 stdev of peer-reviewed study)
Extra gas used in Civic over Prius: (313 – 199) = 114 gallons/year
Extra CO2 produced by Civic over Prius = 144 * 0.00907 metric tons/gallon = 1.306

Extra social cost per year of Civic over Prius (low) = 1.306 * $39 = $51/year
Extra social cost per year of Civic over Prius (high) = 1.306 * $209 = $273/year

Over 5 years, the Prius costs $1600/more than Civic, which is $320/year. Even the high estimate for social cost of carbon ($273/year) doesn’t make up the difference.