Airticket from London to Prague: $100
Taxi to the airport: $250
Leaving for the airport early enough to take the train next time: priceless. (Well, not completely priceless, but about $230 cheaper than taking a cab.)
The good part is that I actually made the flight with two minutes to spare. The bad part is that it would have been cheaper to just miss the flight and buy an new air ticket.
Can you believe it actually costs twice as much to fly internationally as it does to take a cab from the center of London to Stansted airport?