Your costs may be different depending on your cost for electricity.
But I use my hot tub everyday after work and using the hot hot everyday after work costs me $30.00 per month in electricity costs which isn't all that much and it's very nice relaxing in my hot tub after a stressful day at work.
If you use the hot tub more often then your costs may go up a little but but I would doubt it would cost more than $60.00 per month to keep your hot tub heated.