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.