Your emission per unit volume numbers are a little off. A $50/tonne CO2 tax adds about 11.5 cents/liter for gasoline, 13.5 cents/liter for diesel, and 9.5 cents/cubic meter for natural gas.

The larger error is that you were calculating costs based on the introductory $10/tonne rather than the $50/tonne that is being more widely cited, even if it's not scheduled to come into force until 2022.