$300 for "normal" speaker cables sounds expensive to me. $6000 sounds crazy, but who's to say what's a worthwhile purchase?
I'm a bit of a cable sceptic, and could never bring myself to pay that much. Though having said that, I got new silver coated speaker cables today, and I think I heard a slight improvement, though I think I need to hear some more.
They were $100 for 3 metres, the old ones were $40 for the same length.