Here's the methodology.
Let's take the paying WEIGHT that a plane transports from A to B.
The paying weight is made up of passengers, their baggage, and any cargo that the plane carries.
As an example, let's take an Airbus A320.
Typical 2 class configuration is 150 passengers. Take a load factor of 86.6%, so 130 passengers
Consider a trip of the maximum range (3,700 miles), uses maximum 6,400 US Gallons of fuel.
Empty Weight of A320
So, each passenger consumes approximately (200/(26000+7376)*6,400=38 US Gallons to travel 3,700 Miles. (not bad!)