How much does it cost to run a ceiling fan at low speed?

Not enough that you should even notice. Run the fan, be cool and enjoy it. I know this is not an answer in dollars and cents, but it is so very little

That's true. A fan on low probably uses around 20 watts, depending on the fan. Little devices we have plugged in we're not even using can draw more. I leave my fans on low 24/7, unless I need them on high.

Frequently asked ceiling fan questions - Ceiling Fans N More