The cost to run a Window Air Conditioner per month depends on the size of the Window Air conditioner and your cost for electricity.
Small 5,000 BTU window air conditioners typically cost $15.00 to $20.00 per month to run and larger 8,000 to 12,000 BTU Window Air Conditioners cost around $35.00 to $60.00 per month.
If we knew the wattage or size of your Window Air Conditioner then we could give you a more accurate answer.
But usually a Window AC is not that costly to run but central A/C is more costly and can cost from $250.00 to $300.00 per month to keep your house cool.
Window Air Conditioners are usually cheaper to run when you're just cooling one or two rooms in your house instead of the whole house.