dry ice is actually a common cooling method amongst performance overclockers however. There are pre-made and DIY DICE-pots (dry ice) available and they do give superb performance. However they are probably not cost effective as mentioned, compared to say, running a fan, though it's not really *that* expensive. $1/pound is easy to come by for even a modest customer, and that's a fair amount of dry ice really.
Personally, I do 2 things. I buy glaciers at bottom-dollar prices, and I buy shit-tons of algae to suck up the carbon dioxide in my room so that heat escapes more easily.