I don't drink bottled water, but I have had some flavored Aquafina (I think) type things for like $1.50 each at gas stations. My wife sometimes gets plain water bottles much cheaper than they sell these flavored ones in bulk I think. Then the cost of the required flavoring is probably next to nothing. E-
juice flavorings need to be much more concentrated than for other applications. I've heard anywhere from 4-20 times as much.
So yeah, it must be cheaper and you get the advantages of making your own flavor and not having preservatives or other junk.