If you have a few clean and empty water jugs (or better bottle or 5 gallon Homer bucket), you can take it to your local supermarket and fill it up with RO water for around 35 cents/gallon. You're saving money by not having to pay for the container.
And you know the water is pretty much devoid of any salts. So you'll be adding brew salts to create the desired profile for the style you're planning to make. The salts are not expensive and your local grocery store, LHBS, or online sites have all the ones you'll need.
Kosher salt [no iodine], epsom salt, and baking soda are available at your grocery store, while CaCl, and chalk, and gypsum are at your LHBS or online. Don't forget activated charcoal filters (use the RV inline filter that hooks up to your garden hose--about 20 bucks at WalMart) and campden tablets (to deactivate chlorine and chloramines) if you do use your local water instead. I have crappy water (excessively high bicarbonates and high sulfates), so I prefer to just build up from RO water.
IMO, it is a small price to pay for better beer since I'm investing so much of my valuable personal time in brewing--I might as well make sure that the ingredient that makes up 90% of my beer is as good as I can get it. I devote similar attention to the quality and health of the other three ingredients (malt, hops, yeast) as well as optimizing sanitation and fermentation temps.