Not as cheap (per gallon) as your idea, but I thought this was a pretty clever idea

http://cgi.ebay.com/Emergency-Disaster-Preparation-kit-Water-Storage-%20mre_W0QQitemZ230092537998QQihZ013QQcategoryZ3204QQ%3Cbr%20/%3EssPageNameZWDVWQQrdZ1QQcmdZViewItem

Takes the filling the bathtub idea to the next step, because this keeps it clean enough to drink, and you don't have to worry about your drain plug leaking. The only downside to this is they're not really for long term storage, unless you happen to have a spare bathtub in the house. It's more for a quick backup if you have some advance notice of a possible emergency, like a hurricane or ice storm or something.