I think you may still be under what the fans are, but it won't be outrageous.
You can figure it out, by adding up the wattage of each fan (though the ceiling fans are 12VDC, so it may make more sense to just rate your converter). If they have amps instead of watts....Amps X 110 (V) = Watts
(Say that your A/C is 15A.....x 110 = 1650Watts.
with say, $0.10/Kwh.....it's gonna cost you about $4/day....
(not absolutely sure that the math is correct, since $120/month seems a bit high)
Oh yeah, you're not going to be running it 24 hours straight.....:S
So, you should be able to figure the price of running the fans....and go from there!
You'll also want to know what your Kw/hour rate is from the electric company...
Not sure if it's really worth it all.....plug in and stay cool!!:B