What does 90% of the Absolute Best Sound Cost?

This is subjective based on room size, room acoustics, and the audiophiles ability to discern subtle differences in sound quality between brands and components,

for as you go to higher price point equipment, the absolute value of the performance/price ratio becomes smaller.  Differences perceived important to one listener may not be important to another.  That said, from my personal perspective a spend of around $75k should get to 95% of the best of the best.  If you buy preowned you can take 30% off of that spend.  To me, I believe my system is at the 95% performance level at a retail cost of around $90k and a preowned cost of about $45k.  I would have to double that spend to achieve a 2 or 3% improvement and much more because my small home and listening room would need expansion to accommodate the equipment required to achieve improvement.   My home is my limiting factor.  My weakest link now is my server/streamer where I would need to double or triple my spend.  

Interesting question. For the sake of discussion I'm going to use retail prices for this exercise. Obviously you could do much better by buying used equipment, which, like you, I did, but I'm trying to compare apples to apples.

The core of my system is from the mid 1990's (Krell KSA 300S, Krell KRC-2, Thiel CS6 speakers, Velodyne subwoofer). If I take the original retail prices and adjust for inflation those pieces would cost about $35,000 today. I've got several DACs, two transports, two turntables, tape decks, and other gear but I'm going to figure my total system cost with my best DAC, transport, and turntable. At retail (inflation adjusted dollars)  my total system today would have cost about around $60K. For perspecitve, my actual cost is around $25,000 (I bought most of my gear used).

I've been to three audio shows and heard dozens of high end systems. I'm pretty comfortable in saying that my system hits the 90% target of the best sound regardless of price. I've heard a couple of million dollar systems and a bunch of six figure systems and very few of them sounded significantly better than my setup. I've heard far more systems that I thought sounded great but not particularly better than my rig.

A few speakers have stood out - Von Schweikert Ultra 11 ($325k), large Acoras ($200k+), and especially MBL 101E Mk II ($90k). These systems definitely reached a level beyond what I can coax out of my system but at an astromical price. On the other hand I've heard several speakers under $25k that sounded wonderful, if not quite state of the art. One speaker that bowled me over was the Rosso Fiorintino Fiesole (around $21k). I'm looking forward to hearing other models in their line.

I'm at the stage of my life where I could afford a significant upgrade but I'm not sure where to go from here. I'm afraid that I could drop a significant amount of money and a lot of hassle on new gear without getting much benefit.

From my experience I would say that $50,000 spent today easily reaches 90% of the best sound money can buy. 

Based on my experience over decades I'd suggest somewhere in vicinity of $100-150K retail. Costs add up quickly even with the small stuff like cabling, dedicated AC lines, room treatments. Purchasing used can bring the costs down from the retail cost I stated.

Your answer: start with 100% and subtract 10%

In the end, it depends on how much obsessiveness and pathological compulsion dedication and commitment one is willing to bring to the table.

As @onhwy61 rightly pointed out, a purpose-built listening room becomes a necessity if one is to even hear the difference better gear makes. Yes that room can cost $250K+, though it doesn't have to.

I don't have a dedicated listening room and I don't want one. I sometimes listen to music 12 hours a day so I need to do something else at the same time, namely, in my case, work. That supposes, by definition, a very imperfect room, which in turn limits how much better gear I can get while still reasonably hoping to hear significant improvement. But I wouldn't last an hour a day in a dedicated listening room before I got cabin fever, so I'm more than willing to live with what I have as it lets me listen to music for another 11 hours, and at the end of the day listening to music is what I'm here for.