To give you a more accurate answer we would need to know the size of the window ac and how many watts that ac uses.
But lets assume your window ac is a small 5,000 BTU air conditioner that uses 450 watts of electricity per hour.
If your KWH cost is 10 cents and you ran the window ac for 8 hours out of the day the window ac would use 3,600 KWH per day.
At 10 cents per KWH that would mean it would only cost you around 35 cents per day. That window ac would only cost you around $10.50 cents per month to run.
That's a very cheap price to pay for staying cool.
Now if your window ac uses 1,000 watts you would use 8 KWH so it would cost 80 cents per day for 8 hours per day.
That 1,000 watt window ac would cost around $28.00 per month.