There are direct cost of a plane ticket, this is what you pay for the ticket.
However the big two indirect cost are Transportation and Lodging. I am leaving from Frankfurt Airport at 7:00 AM, the subway stops at 2:00 PM in the morning as best I can tell, so the only real option is a taxi. I will go by Subway and sleep in the airport to minimize the indirect cost.
Often there are cheap tickets from New York City or I can fly into London however both of these cities are very expensive for lodging. The cost of the lodging and transportation are an essential cost or should be integrated into the total cost of the ticket. Transportation is very difficult to anticipate, many of the cheapies like Ryanair.com and Easyjet.com are located at airports where it cost a lot of money for transportation to travel too.
Heathrow is easy and on the metro or tube. Stansted, Luton and the other one are going to cost about 30 dollars or 15 pound to get to these airports.
There are many layers to travel expenses, not just calling up the agent and leaving, unless you do not care.