About the cheapest I've found is CCI Blazer, with the aluminum casings. Burns pretty clean, though the last batch I shot wasn't making the slide go all the way back, so the slide wouldn't lock open after the last shot.
Check here:
http://www.natchezss.com/ammo.cfm?contentID=productDetail&ammoGroup=2&a mp;brand=CC&prodID=CC3582
Occasionally, they'll have it on sale for $6.50 or $6.95.
FC