Shortly after I bought my home about 9 years ago I bought and installed compact flourescent lamps (CLF) in almost all my light fixtures. I don't recall exactly how much I spend because it was years ago. I am guessing that I replaced around 10-15 bulbs total and I probably spent $50 to $100 to buy them. The CFL's were more expensive several years ago and their prices have dropped quite a bit.
To many people CFL's may be old news. They've been around for years and are gaining in popularity. Personally I think CFL's are one of the easiest ways to save some money on your electric bills.
CFL's save you money in two ways. First they use much less electricity. CFL's use about 20-25% of the electricity of a normal incandescent lamp. Second they last a lot longer so you save from not having to pay for new bulbs.
I figure that I typically have an average of 3 bulbs on when I'm home in the evening. I probably spend an average of 8 hours a day at home at night. 3 bulbs saving 70W at a time is a 210W savings. If you save that much for 8 hours a day then its 1680 W/hrs or 1.68 kWh daily. Annually that adds up to 613.2 kWh. My electricity runs just about $0.10 per kWh. So roughly speaking I'm saving about $61.32 per year on electricity.
CFL's last about 10 times as long as a normal bulb. The lifespan for a CFL is typically 10,000 hours and a normal incandescent bulb is around 1,000. CFL's can typically be bought for $1-2 each nowadays. You can get a 12 pack of 13W Sylvania bulbs for $20.32 which comes to $1.69 each. Incandescent bulbs would have to cost less than $0.17 each to be cost competitive. Across 10 bulbs installed in a house you'd easily be saving $1 or more a year in lower replacement costs.
Since I bought my bulbs about 9 years ago and I've saved over $61 a year in electricity I've already saved over $549 in electricity. Minus the cost of buying the CFL's, I'm ahead at least $449 already.