LED lightbulbs are expensive, but they last a long time and use relatively little power. One LED lightbulb costs $35 but lasts 60,000 hours, during which time you'd have to buy 60 incandescent bulbs (at a cost of $40) and the difference in the cost of the electricity over the 60,000 hours is $360 for the incandescent versus $12 for the LED bulb. (via a.whole)
Update: Something to keep in mind...the above comparison is a bit apples and oranges because as the page states, the LED lightbulbs have a "reduced light output" compared to regular bulbs. The featured LED bulb only puts out 31 lumens of brightness while a 60 W incandescent puts out 850 lumens. (thx, kevin)