Don't you have any bleach in the house? Can't get much cheaper or more effective than that. Personally, I wouldn't risk just using plain water -- it is amazing how quickly bugs can move into your beer.
I don't doubt for a second that plain water works for some, but not all water sources are equal!
One thing that you want to watch out for no matter what liquid you use is that if you crash-cool your carboys (as I do), the liquid can get sucked up into the tube if you are not careful. If you used plain water, then anything that got in that water could get into your fermented beer.
Not that big a deal as fermented beer is not as susceptible to infection as unfermented beer, what with the now-present alcohol, but still it could throw all of your sanitation efforts down the toilet.