Want to watch the meter? There are two Kill a Watt devices on this page: Safe Home Products - Product Match
One is a bit more sophisticated that the other because it does projections and tries to calculate a real cost.
Electricity is measured in Killowatt-hours or 1000 Watts in one hour. 1 KWH of electricity probably costs somewhere near $.15/KWH. Check your electric bill.
A 100 W lamp is 0.1 KW and if it burns for 1 hour, it uses 0.1 KWH. At $0.15/KWH, the cost to run this bulb is 1.5 cents/hour.
Wattage is only one variable when it comes to lighting. Incadesent lamps (the STANDARD light bulb)are less efficient than Flouresent lamps, so the parameter lumens/watt tells you how bright the light per the power required to create light. LED lamps will be overtaking the standard light bulb. The higher the number, the better. Incadesent lamps also contribute heat to the surroundings as well and not all lamps have the same spectra or color. To further complicate lighting, the eye has different sensitivites to colors.
A standard light bulb may burn 15 times it's rated power for an instant when it starts up, but that doesn't contribute significantly to electricity consumption, but rather the lamp might burn out sooner.
So, turn off lights when not in use. Use Flourescents or LED lamps when possible.
There are 3-way compact flourescents that can be considered too.
For more information see: Fluorescent Lighting Energy Savings and Product Comparisons