chefrp
I suspect that if you asked a dozen accountants what a flight costs you would get at least two dozen replies. Different airlines will account for costs differently and make different decisions.
You have the direct costs (fuel, crew, landing fees, etc) but also overheads (running the head office, etc). You can always look up an airlines accounts and divide the total costs by the available seat km which gives cost per available km. Costs such as handling may be a bit higher for longer distance flights but certainly don't rise in line with stage length so longer flights should have a lower cost/ASKm. The other approach, as suggested, is calculate the fuel cost per sector (hours x tonnes/hr x cost/tonne) and divide this by the proportion of costs accounted for by fuel. This approach would work if fuel costs were stable (of course they never are).
If you look at airline seat plans long haul J class seats may occupy three times the floor space as Y. However the load factor will be lower. Take this into account and you would expect to pay four times as much. I don't know how catering costs differ between the two or indeed total passenger service costs including lounges, etc.
I suspect that the average yield for J compared with Y is more than 4 times as much on some routes although it will obvious vary by time of year. A few years ago I read that all the profit on the North Atlantic came from premium traffic and that Y made a loss (although again this will vary by airline, route, time of year, etc).
One of the reasons for the growth in premium economy is the huge gulf between J & Y. I have read that its profitability is closer to that of J than Y, its good for the bottom line. Looking at fares it seems that many travellers are willing to pay a high premium.
Its interesting that BA are reconfiguring some of their 747s increasing J and reducing Y. They obviously think that the money is in J.
It would be interesting to know how much it would cost to reduce density and if people would pay. Airlines claim that punters won't pay extra, although that may depend upon how tall one is.