Two years ago during this billing period I used 7.43 kWh of electricity. Last year, 8.19 kWh. This year I used 8.03 kWh.
I've installed a number of compact fluorescent bulbs in the past year (but not all bulbs were replaced), so if you take the very simplest interpretation of the data (ignore random variations, changes in use patterns, different major appliances, etc) I have saved 0.16 kWh by installing the bulbs. Use-based charges are about 12.8 cents per kWh (all the charges including generation, delivery, and users utility tax are scaled by usage. The one exception is the "basic" charge.)
That means that I have saved 0.16 kWh * 12.8 cents per kWh = 2 cents this month.
Suppose I was on an upward trend of usage. That means that without CFLs I could have used 8.95 kWh this billing period, so my savings might be as high as $0.12 for this billing period.
I'm happy to be doing my part for the environment, but $0.02 per month savings will take a long time to pay back my CFL investment.
I didn't replace bulbs that have the majority of the use. This is true in part. I have various hanging fixtures which take fan bulbs and I didn't like the look of the CFL replacements a year ago, so those didn't get replaced. Most others did.
Lighting must not be my primary use of electricity. Probably also true. My computer is on all the time and then there's the fridge. A hypothetical 50% savings on 10% of my consumption would only have a 5% effect on my bill.