I can't tell you the actual cost because that depends on the wattage of the bulb and your electric rates.
Turning on a light bulb does cause a slight surge in power until the filament heats up (less than a second). Turning the light on again in 15 minutes will likely use more power than if you left it on that long. Also each time the light is turned on, it's life is slightly shortened. My rule is; if I will be leaving a room for more than 20 minutes, I turn the light off.
All I've written applies to regular incandescent bulbs. I don't know if the new power saving bulbs work the same.
If you want to save on the electric bill replace all the old bulbs that you can with the new ones. For cases where you have to use the old bulbs, you can follow my tips.......or not. 
