Short haul return flight: up to 1000 miles (1600km) each way
Intra-European flights, London - Madrid, cost US$18

Medium haul return flight: 1000 to 4000 miles each way
London to Dubai or New York to Los Angeles, cost US$36

Long haul return flight: above 4000 miles each way
London to Singapore or London - Aust/NZ, cost US$54

If you want to be really through, the best starting point is a website that aims simply to tell you who flies where. Don't risk confusing the issue by going to a website that tells you anything about fares.