Tap water costs 0.002 per gallon. If you filter it, the cost is about 0.25 gallon. Bottle water is about $10.66 gallon. Cost wise, giving it up is a no-brainer. If you are worried about drinking your tap water, test it. If it needs filtration, buy the proper filter.

You can get complete details on how to test your water and get the correct filter by reading Take Back the Tap Guide to Home Tap Water Filtration.