Not really. Box fans use around 100 watts of electricity so you'd have to run it for 10 hours to use 1 KWH.
My kwh cost is 10 cents per kwh so to run my box fan for 10 hours per day it would cost me 10 cents per day.
10 cents x 30 days is $3.00
If I use the fan for 24 hours which I don't it would cost me a little over 20 cents to run per day which would make the monthly cost around $7.00 a month to run the box fan. That's not much at all.
Using fans is cheaper than using air conditioners which would cost around $40.00 per month.