Where does it end?

When I got back into audio in 2014, I felt extravagant paying about $1K for a Rega RP-3.

Last month I spent $4,300 on a pair of *used* speaker cables.
Are you asking the buyers, the vendors, or both here?
It doesn't for most people ! Fun Hobby and depending on your taste in equipment not cheap.
“Last month I spent $4,300 on a pair of *used* speaker cables.”

Well, you didn’t say what brand of cables or details of rest of your system but $4300 is lot of money for used speaker cables. May be rest of your system is say over $100K to justify the four figures speaker cables :-)
There is NO end to this $#!+.
I guess it ends when you quit buying cables for $4300.  It's your choice.
Having a hard time understanding the question. What do you mean, "end"?
Payment pains.. They come they go... Then you deliver. OUCH..

Really want to spend money start changing spouses.

They get all the equipment and you get the bill, your doing fine.


Last month I spent $4,300 on a pair of *used* speaker cables.
You just got started...why do you want it to end? :-)
You have to understand what makes a system sound better to you and there in is the issue with most people.  Comes back to tubes versus SS, copper/silver.  Once you figure out what makes something sound better you don't have to chase sound.  For a few bucks and knowledge you can change resistors, capacitors, AC filter chokes, etc.  Component design such as point-to-point wired, separate power supply, quality of transformers, etc.

Happy Listening.
I don’t know where "the end" is. Not in terms of price. Not in terms of things you can do. Not even in terms of self-serving virtue-signaling posts. (Look how great I am! I can afford expensive stuff!)

I will however venture that when it gets to where you’re paying $300 for 1.5ml of goop in a test tube (that nobody knows what it is, or how it works, and they don’t even make any more anyway, yet it is the hottest thing and everyone who is anyone wants it), with a microscopic brush and surgical gloves required for application, you are still nowhere near the end. But roughly I would say maybe about halfway there. In all respects.
Hopefully, you've upgraded the table/phono stage  as much as those magic cables.

Those cables better be attached to $10K+,beautiful sounding/looking speakers. Nothing gets done without a solid amp $10K+.

Still not cooking with gas.
All that is a waste if the room isn't treated appropriately.

We haven't even tweaked yet, or used a  "proper" LP. 
It ends where you can see the copper. For $4300, maybe gold?
When you are not interested any more.

Okay, you got a turntable and speaker cables.  Now you need an integrated amplifier and speakers to make it work - the end. 
Never ends just keep spending big bucks as its way cool.
It ends when you get where you want to be...unless something breaks or there is a significant advance. $4000 for speaker wires?
I use one of the best speaker wires available, Kimber 12TC. Since I terminate them myself they cost me $300 including connectors. If I put a sock over them I could pass them off as $10,000 cables in this silly market. 
Back in the early 80s, I was using a modified Dyna-70 and a modified Dyna PAS pre-amp in my main system. They sounded really good. I was toying with the idea of upgrading the pre-amp. At the time Audio Illusions was getting rave reviews. At $1100 it would have been a stretch at the time, but doable. So ... I took the trek down the Coast Highway to Santa Monica to visit Randy Cooley’s "Optimal Enchantment."

After the audition, I told Randy that the Audio Illusion piece sounded pretty much like what I had at home. Randy said, "well, let me demonstrate the Audio Research SP-??. It sounded glorious. When I asked what the price was, he said "$3,500. I looked at Randy in shock and said ... "Are you kidding Randy? Who in their right mind would pay $3,500 for a preamp?" Randy smiled and simply replied ... "Guys like you." Ha! Prophetic words, for sure.

By the way, if you are in Southern California, I encourage you to make an appointment with Randy. A cooler guy doesn’t exist in the business. And ... a true music lover to boot.

To simply answer the OP’s question: It doesn’t.

...when you can't hear good no mo...
I found that dissatisfaction with my system and the size of my vinyl and cd-collection ended about when I had to start making mortgage payments
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The harmony is hiding in balancing between things. Try to spend equally on things you like most. My recent dilemma was to buy a new MC cart or new bicycle. Considering that summer is coming and the fact that I already own 5 different carts, the bicycle has won :)
If you are the inquisitive type it will never end as long as discretionary income permits. On the other hand if you reach a point where you can’t believe how good your stereo sounds, leave it alone.
Where’s the end? ... Death. 
It’s a merry-go-round. You can jump off, but you can’t stop it. And you can always get back on for a few more circuits and still end up right where you started, or somewhere completely different. Just try not to fall on your head going for the brass ring.