Well here we go, in a first attempt to answer to my own question, until Threy comes up with something more precise. Papa Hotel tells us that a journey such as JFK/CDG needs 80,000 to 85,000 kg of fuel, but he doesn't specify for which type of aircraft. 85,000 kg is 187,000 lbs and a gallon of aviation fuel (via a google search) weighs 6lbs, so 31,166 gallons are needed. Threy guesses that Air France would be paying around $1 a gallon at JFK. So thats $31,166 (lets say $32,000).
300 passengers on a 777-300 paying a surcharge of $55 each way will net $16,500 (or about half of the total cost of 32,000 gallons). 400 or 460 passengers on a 747 would bring in between $22,000 and $25,000.
Does all this sound reasonable? On this calculation, airline fares include the cost of fuel if it is running at half the present price. Any more than that has to be passed on via surcharges.