You would have to run the 100 watt light bulb for 10 hours to use 1 KWH.
So if you pay 10 cents per kwh it will probably cost you about 8 cents to run the light bulb for 8 hours.
Not too bad of a cost to run a 100 watt light bulb.
The cost though will depend on how much you pay for your electricity per kwh.