You can make a chiller for less than $200, all you need is a mini fridge, drill and bits, some pvc, connectors and tubeing.
Remove the freezer box so that the freezer isnt seperated from the rest of the interior. Drill two holes in the top for your "in" and "out" fit the pvc in the holes leaving 2 inches or so on either side of the holes. Seal the holes with the pvc with some good silicon. attach your connectors to both sides of the pvc. Take between 50-100 feet of tubeing and coil it up, put that in the fridge and attach one end to each connector on the pvc. Then take another section of tubeing and run it to a powerhead and the "in" connector then another section of tubeing from the "out" to the tank...and there you go. The most expensive part is obviously the fridge. But once you have it up and running you can stick a 6 pack in there and not only does it keep your water nice and cool, but you have a cold sixer on hand!
PM me if you want the schematic...I found it on About.com's saltwater DIY section...