From Heathrow's 2014 accounts they got revenue of £2.7bn, made up of £1.7bn from air operations, £0.5bn from retail, and £0.5bn from others (car parks etc).
Their actual operating expenses were £1.6bn, made up of £1.1 operating costs and £0.5bn of depreciation of past investment.
With 73 million passengers, each therefore contributes on average £37 to revenue, made up of £24 air ops, £7 retail (one coffee plus doughnut at prevailing rates), £7 others.
That's a huge margin for a regulated monopoly. Quite why R3 has to cost £18bn is an absolute mystery to the construction industry, but is a handy figure to justify jacking up charges in advance. Manchester opened full length runway 2 in 2001 at a cost of £172m, and although 15 years have passed and the Heathrow scheme is more than just a runway, a cost which is more than 100 TIMES as much as Manchester paid is just ludicrous, given that much of the land purchase has been steadily made over the years, including whatever houses in Harmondsworth etc come onto the market (they are then rented out short term), and thus are already in the sunk costs.