RO water at my local grocery is 30 cents a gallon, Walmart is 33 cents. The thing about these sources is that they're only RO and therefore won't be as good as having your own RO/DI unit. Plus, you have no real way of telling how often they change the cartridges and membranes in their units. I used RO water from my local grocery for over a year without
too much trouble (had some problem algae in my 55) but just bought an RO/DI unit after xmas and have already found that the convenience of having it definitely outweighs everything.