The difference between cheap and expensive bottled water is taste (and texture).
The reason most people don’t taste a difference between tap, cheap, and expensive bottled water is that people, especially in the West, drink water cold and mainly after eating, both which largely cancel out differences of taste.
If you were to regularly drink water at room temperature, as I do, you’d realize that (except for the sense that it seems to always be more aerated than most bottled waters) tap water isn’t that amazing; it leaves a chemical aftertaste because tap water sources are treated with chloramines, and in part because they contain certain quantities of bacteria, tylenol, fluoride, caffeine, unspecified chemicals, and arsenic.
Cheap brands like Aquafina, which originate from “municipal” or “public water” sources aren’t far off from tap water (they, in fact, are tap water – just sort of filtered, badly). Brands from spring water sources are generally better and more expensive.
Nestlé Pure Life is the best tasting most accessible water for your money (I’ve heard Mountain Valley Spring Water is better, but I’ve yet to try it). In my opinion Pure Life is better than Smartwater, FIJI, Aquafina, Evian, Dasani, VOSS and all no-name brands I’ve tasted so far from Canada, America, and Thailand (most local brands of water I’ve found in Thailand just go through a process of reverse osmosis).
Blind taste tests I’ve found online agree with me on this, except on VOSS – most prefer VOSS to Pure Life (but there’s a bit of skepticism on my part because most of these tests originate from California, and rich Californians love VOSS).
Pure Life tastes cleaner, in that it leaves no aftertaste and it’s smooth and gentle to the point that you’d think there were no minerals in it (but [like with bacteria, tylenol, fluoride, caffeine, unspecified chemicals, and arsenic] you won’t reasonably taste these minerals).
I don’t expect you to believe me though, no one ever does and I wouldn’t either, so do as I have – buy any and all brands of water casually over a course of two years and never drink any of them chilled (I regularly drink water at a room temperature of approx. 25°C), you’ll taste the difference.