Law of Diminishing Returns; where does it kick in?

So much depends on what you want in a system. Getting that extra little bit definitely costs quite a bit more. With speakers, if you want that big, live sound, with lots of air movement, it is going to take a big speaker, which in turn is going to cost more money than a smaller speaker. Is the sound worth the difference in price? That would be for you to decide.

If you spend $80K for a BMW, is it 4 times as good as a $20K
Honda? For some people, it is definitely worth the money, but it is also relative to what we earn.

This is not an audio only issue, it is relevent to most things; clothes, cars, women, etc. :)

Based on used prices (45 cents/dollar) I'd say full range speakers at 8K; amps at 5K; preamp at 5K; cables at 1K; CD at 6K; TT at 3K. So for about 35K in my estimation you have a high quality reference set up with preferences in intonation/timbre/presentation rather than weaknesses...

The rule of doubling is my rule of thumb. Noticeable differences occur when you DOUBLE the list price of a component. Since this is a geometrical progression most people reach a plateau very quickly! Please note that this is a VERY crude scale, and has much less of an application for speakers and cables.

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.

Mr. Bomarc is quite right in pointing out that this is based on a false premise. Mr. Classsicjazz proposes that up to approx. $78,000.00 MSRP, you are still getting value for each additional dollar spent: I don't think that the said law kicks in at that point, I think it has then reached the point were it has been kicked to death and trampled for good measure. I remember years ago going through the October issue of Audio magazine and fantasizing by building the greatest system on earth by adding to my dream list the most expensive component in every category. Harmless fun, I guess, but not the way to building a realistic hi fi system to live with. It seems to me that a good number of participants to this forum appear to think along such lines. Any attempt at drawing lines at which one should simply fold his hand and walk out will be met by the bouncy rhythm of audiophiles jumping up and down claiming that every last microgram of "improvement" justifies any increase in price. The strange thing here is that we are not dealing with percentage increases but with multiplying the price by factors of two, three or four times and yet there are still those who contend that the perceived added benefit is justified. With no objective guidelines on how each component should perform, there is no way to set a dollar amount at which the whole thing becomes abusive. If you do as cable manufacturers do and claim that the whole thing is a question of aesthetics or perception, then the sky is the limit. I have no wish of leaving the bang for the buck brigade, so my priorities and price points are pretty well established in my mind and they are a lot closer to Mr. Bomarc's. Happy holiday season, one and all.