The Cockeyed staff have created a listing of a bunch of liquids available in normal or specialized stores. There you can see the prices from water (about $0 per gallon) to penicillin (round $300 per gallon)...
Well, if you're not picky about contaminints, ameobic dysentary or salinity, I would imagine water is $0 dollars a gallon. Once you actually want to not get sick from the water the price goes up.