While compact fluorescent (CF) light bulbs start out costing more than regular light bulbs, they use far less energy - only about 1/4 the electricity of a normal bulb.   On top of that, they'll last years longer.

A basic guide is that you can save $10 a year in electricity cost for each 100 watt bulb you replace - this includes factoring the extra cost of the light bulb and the longer life it has.

So... just replace 5 100w bulbs and you've got your $50 bucks!