We're talking about best sound possible from 2 channel audio, which in my opinion is able to sound very good, but is fundamentally limited by the format.
From my perspective, and from my limited experience I'd say a carefully selected equipment setup could get you 90 percent of that potential for about five thousand dollars. This is assuming you want accurate, uncolored sound that doesn't extend too deep in into the infra-bass, and isn't trying to fill a large room with very high sound levels.
If you want boutique sound, I have no answer. It totally depends on what sort of effect you're looking for.
I'd say $50,000 should be able to get you direct sound from the speakers and electronics that is indistinguishable from perfect, in terms of accuracy, to the human ear (for that format.)
The catch is I said direct sound. Excellent $50,000 systems with very wisely chosen components can all sound different. That's because perfect hasn't been defined yet in terms of dispersion and the resulting indirect room reflections. So people may still have their preferences between these systems as played in various rooms. And some people may be perfectly justified in their preference of a $2000 system over a $50,000 system because they like the directivity of the $2000 system's speakers more.