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.
What does 90% of the Absolute Best Sound Cost?
Like many things, I have come to believe that the cost of incremental improvements in audio come at exponentially increasing costs - e.g., big improvement from $5K to $10K, less so from $10 to $15K, etc. All of us have our limits regarding what we can/will spend to achieve our best possible/practical audio experience. So, a couple of questions that I am sure are at least somewhat subjective.
A. What does it cost, in terms of components, interconnects, and direct furnishings (e.g., racks, isolation pads, surface room treatments, etc.) to achieve 90% of the absolute best sound possible?
B. What % does $50,000 get you?
FWIW, my setup is at about $21,000 actual cost ($32,000 original retail) and I am really happy with it right now. All of my incremental spend for the next couple of years is going to be working the room itself. Looking forward to your perspectives!