The cost to leave the porch light on all night would depend on the wattage of the light bulb. My porch light led bulb uses only 9 watts so it would cost very little to leave it on all night.
The bulb I use for my porch light consumes 9 watts so if we take 9 watts and times that by 8 hours per night we would get 72 watts per night.
72 watts x 30 days in a month would equal 2,160 watts of electricity per month so assuming your electricity cost was 10 cents per kwh you would pay roughly 22 cents per month to run your your porch light all night for 30 days.
If you left that same 9 watt led bulb on for 24 hours per day at 10 cents per kwh you'd pay 70 cents per month to run that light bulb or $8.40 per year.
If you only left the bulb on for 8 hours per night you would pay $2.64 per year to run the light all night.