Your question contains a false premise, I think: that there's some real relationship between price and sound quality. There is, today, some wonderful stuff at very reasonable prices, and some really bad stuff with price tags in the stratosphere. And given how much personal preference plays into this, it's quite possible that price and quality are random for any given consumer. That said, I'd offer a rule of thumb for three items:
Speakers: never.
I/Cs: $10/meter
Cables: $0.50/ft.
Beyond that, it's all personal preference.
Speakers: never.
I/Cs: $10/meter
Cables: $0.50/ft.
Beyond that, it's all personal preference.