What does 90% of the Absolute Best Sound Cost?

Interesting thought experiment, but as the widely divergent answers show, an impossible question to answer because the premise is flawed.
The starting place would be defining exactly what constitutes the “ absolute best sound possible.“  if it’s even possible to define that, it would likely be different for everyone. And if you could create that mythical standard, most of us on this group are probably over 60 and wouldn’t have ears to be able to discern whether it had been met or not anyway. It’s interesting to think about, however, and maybe leads to a better question which is whether you have achieved something you find really enjoyable for the money you are willing to spend.
 

Problem with price comparisons is not every dollar goes to sound quality. For starters, you have to adjust for dealer and distributor markup. Then add the large overhead and marketing costs for large companies. Fancy milled cases also add a premium as do fancy flight cases and white gloves. And I suspect a markup is intentionally added for audiophiles (and reviewers) to take the product seriously.

On the other side of the coin there are products that are designed well and have low prices due to direct sales and efficient manufacturing. Take the Schiit Yddgrasil DAC. For $2,500 not only would you get "90%" of the best of digital playback but you will be hard pressed to find any DAC that surpasses it. It is a superbly engineered product with no dealer/distributor markup, no large overhead or marketing costs, and no fancy casework. All it lacks for audiophiles to take it seriously is the Audiophile Markup.

How much to spend depends on  how much more information one can extract from the software collection (CDs, LPs, etc.). Everyone is limited by their collection and can do only so much for further improvement without running into point of diminishing returns.

My current setup cost ~$30k for my equipment based on late 90s early 2000s retail prices. Not sure how much those would be now, but that figure include only a fraction of the fortune I spent on NOS tubes! Some I brought brand new and some in the used market and paid less than the original price. There is another batch of equipment and vacuum tubes in the storage worth probably another $8-$10k.

I have the bug and there are times I think I want to make changes. Then I come across some modern recordings and my jaw hit the floor!! I can hardly find any shortcomings/faults. Very recently, I heard Mozart Piano Concerto #26 on NPR via my HD radio. I simply loved the presentation, sound, and the whole arrangement. Then I brought the CD, from Bridge Records. It was performed by Vassily Primakov, piano and Odense Symphony Orchestra conducted by Scott Yoo. I always thought I can pay a higher price for a better DAC and that improve "jitter" and other aspects But that turn out to be nonsense when listening to this CD. I have two DACs, AR DAC2 and DAC3 and both DACs played this CD remarkably well and very musical.

Another example is Haydn Cello Concerto # 2, again heard it first on NPR. Then I brought the CD, Truls Mork, cello with Norwegian Chamber Orchestra conducted by Iona Brown. Again same results. So in short, how much improvement one can get by spending more money depends on his/her software collection. Needless to say these modern recordings are much better than older recording. In my humble opinion,  the differences between a 10k system versus 30k or even 50k will be hardly noticeable with newer recordings.

This is not to say digital playback cannot be improved. You are limited by 44.1 kHz sampling rate and 16 bit word length. One can definitely improve on jitter, channel separation from 100db to 110db or higher, higher order digital/analog filters, etc. I am not saying I can hear these differences, but others may. Same goes for LPs. It is an inherently faulty system. Higher priced turntables, tone arms, cartridges etc., may be rewarding. However, one has to keep on mind about the price point of diminishing returns. Another good and rewarding investment is the listening room. Even a lower priced system can sound like a more expensive system in the right listening room. Such is life.

I have right at $50k in my system.  Seems like a lot, but people spend considerably more than that on pickup trucks and large SUVs. I get a lot more enjoyment out of my audio system than I do my vehicles.

I have no idea what percentage of sound quality I get compared to the ultimate system.  It sounds great to me and I don't think I need the ultimate system. 

If you eliminate the hype, the sales pitches and the peer pressure, you'd be surprised at how good your system can sound.  Once you step outside yourself for a moment, you realize that it's all rather ephemeral and like already mentioned, relative. 

It's like all addictions in that there's that reward at first listen, and then it fails to maintain the high, and then it's on to the next hit. That's not being a lover of music.  

So, what does $50K get you? Sound that you'll enjoy or even fall in love with. The question is, will you allow it to as long as you have it, or tire of it because, reasons. 

All the best,
Nonoise