I recently moved to a new apartment and saw it was a good time to change all the lights to CFL. I did a total do over of all the light sockets and lamps, even going to the extreme by including the candelabra, the bulb in the fridge, to the light above the stove.

My former one bedroom apartment averaged $100 per two months (I live in a municipal electricity area where it's billed on a bi-monthly basis). My new two-bedroom apartment now averages $40 per two months ^

Total cost to invest in CFLs: $40 (candelabra CFLs are quite expensive)
Savings from the first bill alone: $60

And these CFLs are to last 10,000 hrs or more, the savings are well worth it!!