What is the correct cost for a flight leaving in 3 hours with an empty seat? What is the correct cost for a scheduled flight leaving in 2 months with no seats sold yet?

Tickets aren't the same price for everyone, and planes fill to variable levels. Plus there are addons like luggage fees and beverages that have a huge markup. What is the best way to solve for that?

Besides, it averages something like 53L of fuel/passenger to make that trip. Hardly necessitating £500.