try a fan blowing acroos the top. r u using the glass tops, this may also aid in the cooling but may have a little more evaporation. I have a 180 also with 785 watt mh and mine does not go above 80. I bought the fan at walmart for 6.99 and it is wall mountable. just a thought, as I dont think there is anything as a cheap chiller. unless you look up ghetto chiller in the search page. it is actually a good idea and would probably work..
you can pick up a couple of 120MM pc fans and hook them up to a ac adapter, Mount them into your canopy blowing across your water, this will bring you temp down a few degrees.
There are a couple of places to get chillers and also a few plans out there for DIY chillers, do a google i bet you will find some info!
I dont have a/c in my home and i decied just to let the temp in my tank fluctuate this summer. Seems like a natural thing to me, the ocean temp is not constent. My corals and fish have been fine. I'll probley get ragged for this, just do what works for you. Its like it goes up to 85 in an hour that would stress stuff out
Inexpensive and chillers do go together, I watch a local board untill a used one went up for sale. Half price and been working for 6 months. Also make sure that the exhaust side is open, not inside the stand.
Dewayne