Chillers cost $300 to $400 even for the cheapest and smallest models. Like Brian said, they work like an air conditioner: they remove heat from the tank, and exhaust it back into the room. So your room will get heated up by the hot air it blows, which makes your tank warmer, which makes the chiller work even harder. They are effective if you can vent them to the outside; however.
It is better if you take a clip-on fan and have it blowing across the surface of the water. This can be difficult in closed-top tanks, but it's still better than using a chiller, IMO.