I can get a 4-pack of light bulbs at the 99-cent store. Back in 2000, an article in Slate (http://www.slate.com/id/88871/) showed how you would save a whopping $26 in 3 1/2 years by changing out all your incandescent bulbs to fluorescent. That's about $7.50 a year - less than the price for a movie ticket.
California legislators - please leave me alone!
That chart's pretty old. You can get a CFL for under $5 these days. But for comparison let's use that. A cheap incandescent might last you 750 hours. You've already bought about 10 bulbs for the one CFL (8,000 hour life) at a price of about $2.50. So just bulb buying you're already up to half the cost of a CFL.
Now we have to account for your other $2.50. 8,000 hours of incandescent at 100W is 800 kHh. 8,000 hours of approximate CFL is 184 kWh. Difference is 616 kWh. Depending on your electricity cost, that can be anywhere from $37 to $234, with an average of just over $65. Now subtract the $2.50 and you get about $63.
That's $63 per bulb over about five years (using it about 5 hours a day), or about $13 saved per year per bulb. How many bulbs do you have? I've replaced (very rough estimate) 800W worth of incandescent with CFL, which puts me around $100 per year saved. That's like running my electric water heater for almost three months for free.