The answer is the length of a piece of string, give or take 10% multiplied by the cubic volume of the listening room.
But yes, as a music lover all I need is a $200 bedside CD / radio. To which I am very happily listening to right now as I type. But as an audiophile where I want to immerse myself in the emotional listening experience, things that I once might have deemed as "diminishing returns" suddenly become highly significant. And you can even get to such a critical stage of tuning that changing a $2,000 power cord for a $2,200 power cord of a different brand can be perceived as a make of break sonic decision.
The only rule that I believe hold true as regards price range is that less expensive gear should represent a sonic sin of omission in comparison to more expensive gear and that any system - in any price range - that has any offensive qualities when tested with a wide range of source material should be considered to be a failure.
As for what I know would make me genuinely happy as of today, well there wouldn't really be any change at all from $100K and even then I'd be compromising as compared to what I know is possible. I can achieve a completely inoffensive sound for $20K and an inoffensive headphone based system for only $5K. But those systems are simply incapable of delivering the full experience that I know to be my personal "benchmark" (which btw is still some way below what I might hear in the concert hall).