It depends which $300 cable and which $3000 cable you're talking about. There are some right good $300 cables, and I have another threshold called "ah can has no monies" that prevent me from even trying $3000 cables, so I couldn't honestly tell you. I also have another threshold called "ahm def", which keeps me out of golden ears territory, but I do honestly believe that there are people who can hear better than I do, and to them, there are certainly better cables at higher price points that they are willing to pay. To me, once you get past a certain point, I stop being able to tell them apart -- they're not even different. I don't see the point in me paying more than that. But I do see the point in someone else paying that, if they can hear a difference.
And yes, you're right, once you get into stratospheric prices, there is a certain amount of truth to the fact that they stop being so much "better" and start being just "different" -- Cardas has their house sound cables such as the Golden Reference, and they have there less house sound cables such as the Neutral Reference, just to cater to different tastes.
Also: UsedCable.com -- in addition to the usual suspects, audiogon, etc. Audiogon is a favorite for cable flippers, so the selection will be better there.