Leaving a light bulb on all day costs very little especially with the LED bulbs that are most commonly used today.
An average led light bulb uses around 9 watts of electricity for each hour of being on.
So in 24 hours that 9 watt LED light bulb would use 216 watts of electricity for 24 hours of being on.
So in a month that LED light bulb would consume 6,480 watts of electricity in a month if left on all the time.
So if your electricity costs is 10 cents per KWH that light bulb would cost you around 60 cents per month.
That is very cheap and not even $1.00 per month to leave an LED light bulb on all the time.
I have one light I just leave on all day and night and it doesn't cost me very much on my electricity bill per month.