Chillers seem to just be one of those things that there aren't too many good DIY plans for. I'd say the biggest difficulty is that titanium is the only reef safe metal that doesn't corrode in salt water. The bar fridge mods have been done a few times but the compressors in them aren't really designed for the high temps and continous use so will most likely burn out anyway. As for the costs you'd still need at least a temp controller which is a few bucks anyway.
Rory says it all. Its not pleasant to do but it makes a lot of sense to buy a commercial chiller that has been tried and tested. I do not have experience but think this apply's more to us ou's with the smaller setups,