No, the question should be: how many $$ in production cost does it take to make a gallon of “petrol from air” vs. the production cost of making a gallon of petrol from fossil fuel?
Of course, the Btu value of one gallon of the “petrol from air” must be equal to the number of Btus in the gallon of “petrol from fossil fuel”.
One should also include in the equation that in order to make “petrol from air” the production process will have to use energy from fossil fuel.
No, one can use energy from a nuclear power plant or even windmills or solar panels. (So this process gives a way to power your gasoline burning car with nuclear energy.)