How much do you have to spend?

on a serious note, I think the 5K used number is very true. I usually end up in the range. Once in a while I buy a 4K component (2K used) but then sell and happily swap it with something for half of that.

Difficult. There is no linear relationship between cost vs. quality; cost vs. enjoyment sometimes moves in the "other" direction, and there are complex interactions / dependencies among components of a system.

We each arrive at our internal sense of what is a lot vs. a little for a given component type after experimentation. Then it changes over time. Are you counting MSRP, or willing to explore used markets (better)?

In my present incarnation I’ll splurge most on amp, splashing 20K+ (used) for a big-ass VAC tube amp. I don’t know how to get that sound for less $ yet. I’ve also been equally happy with "just" $2K - $4K tube preamps and phono stages (used), but I’m still quite choosy about its flavor of sound from model to model. For speakers, Tannoys in the $5K - 15K range (used) make me pretty happy. Cables I’ll still splash out a few K for silver speaker cables & interconnects, but I generally care a whole lot less about power & tweaks these days - to the point of almost ignoring them.

Room acoustics has minimal focus. With the controlled dispersion of Tannoys and my seating distance, the room is much less of a factor (with proper setup). I care a whole lot more about "satisfying" bass than accurate bass, which makes things a lot easier too. 

Here we go with the dollars equal quality thing. As if there was some linear relationship between money and quality.

inna: I really have to wonder about your $10K figure for interconnects and power cords. Although I'm not a total cable denier, I am somewhat dubious about the benefits of spending that kind of money for any meaningful improvement.

I was absolutely blown away by the sound I got from the following setup:

Ascend Acoustics Sierra LX (Used - $1100)
Nuprime DAC-9 (Used - $500)
AIYIMA A07 with the bigger PS (New - <$150)
Wiim Pro (New - $120)
Sony UBP-X800M2
Interconnect, digital and speaker cables - Mostly a combination of Belden and Mogami pro cables (New and used ~$150)

The Sierra LX is an amazing pair of speakers that can scale up quite a bit with upgrades upstream. I think the sound quality would have gone up substantially if I had swapped the Aiyima with something like the Buckeye Hypex NC502MP 2-Channel amp ($750). I have since upgraded the DAC/Pre, changed the amplifier and added a good power supply to the Wiim streamer. 

OP "How much do you have to spend?” 

to achieve what, respect from millionaires? you can’t win that poker, since there always will be another $100000 power cable, with “review”, available, claiming …improvements in SQ! grocery shopping with Louis Vuitton bag is more successful? LOL 

being audiophile, analog circuit designer, for many years, I would agree with richgeova’s comment, who found his/her way to achieve desirable SQ!