I'm in the UK and although I could do with one in the summer months, i manage to get away with a pedestal fan pointing at the top of the tank which will reduce the temperature by 3-4c if needed.
The only reason i don't have a chiller is because of the heat output they produce into my living room, and for the few months its needed I can get away with a fan. For the moment anyway!
Any time you can get by with a fan instead of a chiller you should do so. Chillers are huge energy sinks, and as others have pointed out they simply add heat to the room they’re in, so unless you have the chiller unit outside, you cool the tank at the expense of heating the room.
Other things to consider are changing the plumbing so the pump is external, and switching from Halides to T5s or LEDs. (I know lighting is akin to religion, but halides and to a much lower extent T5s do heat up the tank. If you are considering getting a chiller, you can probably pay for new lighting with the energy savings.