Bottled water is relatively inexpensive unless you decide to use something like Evian for example, however it will never be as cheap as city water from a faucet. Although you cannot use this water right out of the tap, it is very easy to rectify. There are 2 ways to do this,
1) Chemically treat it with a commercial treatment available at most , if not all, pet stores. Zoo Med's Reptisafe is one example. Just a few dollars will treat gallons of water to be treated as needed.
2) City supply water can be "cured" to evaporate the chlorine simply by leaving an open container (small bucket or jug) overnight, preferably 2 nights. Chlorine will diffuse naturally without the presence of UV light (any amount of sunlight), hence the over night references.
I don't know what size corn you are anticipating, but hatchlings do require comparatively less water than an adult, however the water dish I recommend be large enough for the snake to get completely into, and of course out of, as it desires. This can be quite small for a hatchling so a full gallon of bottled water will last you months. Since these bottles have no "preservatives" in them and commonly contain high levels of bacteria you will be wasting a lot of that water.
Having said all that it ultimately comes down to your preferences.
I hope this helps some and welcome to corn snakes.
