The cost to run a hot tub per month will depend on your electricity costs per kwh and how often you run the hot tub to heat the water.
If you use your hot tub for 30 hours in a month then the hot tub will cost you $25.00 to $30.00 per month in electricity which isn't that expensive to relax in a hot tub.
The more you use your hot tub the more it will cost you in electricity but rarely does it cost more than $50.00 per month to operate a hot tub which still isn't that expensive.