Hi katiesara - Soda water is just plain water (mineral or tap) with excess carbon dioxide forced into it.. you can do this yourself with a Sodastream or similar. If you put a jug of tap water in the fridge for a couple of hours, the chlorine will have evaporated and it's composition will be indistinguishable from the bottled stuff. The cost will be about 1/100th... more like 1/1000th. You only need soda water if you like the bubbles.
ralph, the first part of your answer is correct. Soda water is ordinary water that has had carbon dioxide forced into it under pressure. The second part though is not exactly true. If you put tap water in the fridge you get cold tap water!
It's often said that it also has sulphur added, but i drink a lot of it and am allergic to sullphur, who knows. It doesn't have the goodies and the baddies in that tapwater contains.