Absolutely the fan will cost less. The AC involves a fan, a compressor, etc so it is much more demanding of electricity than AC. The months we have to run the AC in our small office at work the electrical bill is significantly higher than if the temp stays at a point where we can get away with just using the fan. At home, we don't have AC but use fans (ceiling fans and stand alone fans) and our elec. bill is way lower than our friend who lives alone in a smaller house but uses AC.
The recommendation is to only have fans on in rooms you are in, although I have admit that when it is really hot I like to leave on the fan in my BR because it feels better right when I walk in (trying to break that habit because it is probably more psychological than anything

)