So long story short.
There is no measurable difference between inexpensive and expensive wires of a similar gauge. This translates to no measurable sound difference coming from the speakers. This means that in a properly setup double blind test, you can't hear the difference between cables unless you are psychologically creating the difference.
With that said, the psychological sound difference with having an expensive cable may be enough justification for the cost. When it comes down to it, as the owner of a high-end audio system, what really matters is how it sounds to you. If you decide that spending a large amount on speaker cable is the best upgrade to your system, so be it even if it is just psychological. In reality, you are probably better off spending that money upgrading something else (equipment, speakers, room treatment, etc) but if you truly believe that speaker cable makes your system sound better, more power to you. While you may psychologically think your system sounds better, this doesn't mean there is a quantifiable difference in actual sound. I think there is enough scientific and DBT results to attest that this is true. However, many of you want to justify your purchases and/or want to sell expensive cable/expensive items using cable.
The mind is amazing at creating differences it thinks should be there. For example, one could put the same cable in a crappy jacket delivered in a trash bag and tell you it's worth 1.00$ and another with a stunning jacket in fancy packaging that is told to be worth 1000$ and I guarantee those of you who believe in the benefits of high end cable, will think the stunning jacket/fancy packaging cable sounds better. Do I blame you? No, because it probably really does sound better to you, but at the same time, it doesn't sound any better. We like to justify our purchases in every way possible.
If stroking your mind/ego and buying expensive cable floats your boat, so be it. I honestly hope that you think you can hear the difference.
If you believe that you are the golden eared prodigy and you really can hear the difference, enjoy your million bucks from the challenge, and wrap yourself in your expensive cables at night.
If you are the science minded type who doesn't believe the psychological difference, buy your Monoprice 12g, and save some money.
For all the arguing, scientific facts, what company uses what cable, and what high end audio engineers say, you aren't going to be able to argue through what someones mind is telling them they think they hear. I wish I thought of selling high-end speaker cable 25 years ago