The cost to run your portable air conditioner per month will depend on BTU rating of the portable air conditioner as well as the wattage.
Your portable air conditioner should have the watts required to operate it on a sticker or somewhere on the casing which may be at the bottom of the air conditioner.
But portable air conditioners use from 750 watts to 1,000 watts per hour and some may even use more watts depending on the size of the portable air conditioner.
If your portable air conditioner uses 1,000 watts and you run the portable air conditioner for 10 hours you would use 10,000 watts per day.
If your KWH cost is 10 cents per KWH then it would cost you $1.00 per day to run the portable air conditioner which would come to around $30.00 per month to keep your home cooled down with the portable ac.
That's not too expensive and is a small price to pay to stay cool.
However I find that portable air conditioners in my opinion aren't as efficient or work as good as a good window ac or central air.
But they do work somewhat.