I disagree as it seems highly unlikely that someone would pay money for something they expect to provide an enhancement and not expect it to work. The "hope" that it will enhance is correlated with the pleasure in the brain. Moreover, confirmation bias is not ordinarily found in an experiment that focuses on empirical data. It is found in pseudo science that seeks to pass itself off as veracity. Much like your skilled listener experiment-- you can't control variables properly. There is your logical fallacy. You can't quantify listening skills, just as glupson described above. You can give hearing tests to determine the frequencies people can hear, but you certainly cannot attach a number to what they can hear when listening to subtle music which is comprised of always changing frequencies from multiple different instruments.
The degree to which confirmation bias may effect someone is, as I said up for debate. But even after a failed expectation, they certainly are not going to say, "I am going to find another cable that does not live up to my expectations," (Popper) rather they will look for one that meets their needs/hopes. As such, they will, until the second they make up their mind, be pre-disposed to a positive outcome. How much so is unclear.
Attaching value, $500 vs. $5000 is a highly personal position that you can not possibly know except for yourself. How do you know what sort of disposable cash flow "undecideds and newbies" have, and how much value they place on it? You can't, which is yet another logical fallacy. Also, an assumption that I am a "newbie," your use of the word and, is based upon what? My number of posts? There is your slippery slope.