People usually claim that London taxis are prohibitively expensive and that driving in NY is impossible, however these conclusions wouldn't be the same if they were based purely on relative costs:

Suppose you lived two miles from the center of New York City or London and wanted to go to the town center for a one hour lunch by cab, subway or car, how much would the travel cost:

Cab:
London: $27.60
NYC: $15 [54% of London price]

Subway:
London: $8
New York: $4 [50% of London price]

Car:
London: $21.50
New York: $2.75 [13% of London price]

This is based upon the following data:

Cab:

1 mile cab ride (with 1 minute waiting time), excluding tips:

London: $6.90 (noon), $9 (midnight)
http://www.transportforlondon.gov.uk/pco/fares-detailed.shtml

New York: $3.70 (noon), $4.20 (midnight)
http://www.ny.com/transportation/taxis/

Car:

Toll for driving in city center:
London: $14
http://www.cclondon.com/

New York: $0.

Gas price per gallon
London: $7
http://www.yrl.co.uk/tony/fuel2/fuel2.html

Gas price per gallon
New York: $2.30
http://www.newyorkgasprices.com/

One hour, on street, metered parking:
New York: $2.50
http://www.parkingticket.com/about/press/release_040403.asp

London: $7 per hour
http://www.londontown.com/London/Public_Transport

Subway:

Single peak hour subway/tube ride within center:

London: $3.45, zone 1 ($4, zones 1-2)
http://www.tfl.gov.uk/tfl/fares-tickets/2005/single/tube.shtml