I was going from Marylebone Station to Waterloo Station on the Bakerloo Line a couple of Saturdays ago, when I bought a ticket out of a machine for L4.00, or $8.00; by contrast, a similar trip in Washington, DC would be $1.35.

The good news is that, if you are going to be using public transport in London, you can get the aforementioned Oyster Card, or you can buy a travelcard out of a machine--a travelcard good for London zones 1-2 is L5.10 ($10.20) all day, and for all six London zones is L6.70 ($13.40) per day. If you are arriving at LGW, you can go into the train station, go to one of the machines on your right, and purchase a combination one-way ticket and Zone 1-6 travelcard for a price not much more than the one-way ticket into London (which, on the Southern or First Capital Connect services, is L11.20--the non-stop Gatwick Express is a few more pounds one way). The Southern service offers great value, particularly on their routes that only stop twice--at East Croydon and Clapham Junction).