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.
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 :-)
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. 
Okay, you got a turntable and speaker cables.  Now you need an integrated amplifier and speakers to make it work - the end. 
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.

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.
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.
It’s a hobby. Like many other hobbies, it only stops when one is satisfied or no longer has interest in spending any more money due to lack of curiosity. If one is happy from a plug and play and Fire and forget perspective, and does not wish to experiment or test other combinations of equipment, then that individual is apt to not to explore any more expenditures any more. Good to go. For others, that is a different story.

If that individual is like me, and I will apply my car mod analogy here and just call it mod fever, then spending money on something that may or may not give a performance improvement is perhaps within ones DNA. I mean why take a perfectly good car and add a blower, or twin turbo, headers, adding carbon fiber, cold air intake, or tune, just to gain some more performance. You gain street creds having a faster car or fast looking car. When I added a full Katech racing system to my Corvette Z06, I knew I had passed the line of civility and started racing regularly at the racetrack. So in that instance, I needed better performance to compete against other exotic sports cars at the track. Does one gain better system performance when doing system mods? It’s seems speculative by some, validated by others, real use test cases. 

Again, it is a hobby, we should not question why anyone would want to spend money on things that another wants to experiment, use, or test on their system. Yes, some prices are jaw dropping, but, for the folks who have the resources to purchase such stuff, why question that. My friends drive Exotic sports cars, and my GT350 is part of our car club, but, these are friends, and not people who value you for your worth....so, when applied to our audio hobby, guess what, who’s your friend, someone to call you out for spending money in XYZ, or someone who is willing to understand the stance or position you have taken to gain performance? Friends may laugh at you when you tell them how much you spend on this hobby, but, these are still your friends.

When I added turntable mods, more motors, 20volt dc quartz controlled motor controller, and 80MM platter, I was incrementally increasing performance of my turntable, despite it already being able to spin records in its previous state. These performance mods add some level of performance or system improvement. The only way for one to know if something will work for their particular needs, is to experiment. Hobbyists tend to find ways to spend money on things within their discretionary income levels to experiment with or add an upgrade due to reports from others on forums such as this. I browse sorts car forums and the same type of people surf those forums as well. Some leave their cars as is, others are prone to mod something.

As hobbyists, the spending stops when one decides that they are okay with what they have and no longer have a curiosity to explore other system combinations, or discretionary income tapers off. It seems to me that mod fever is something that traverses many hobbies. In my other hobby, a GT3 may be part of my stable next year. Did I suddenly disparage my fellow GT350 Voodoo engine brethren? Not! At least not with my friends. We love the sound of revving engines and shifting gears as we go to Cars and Coffee, or for cruises on the weekend to explore wineries or take day trips. Both of these hobbies consume a lot of time. When not driving, I listen to music on my modded audio system.

I totally agree with audioquest4life. I've had a few hobbies over the years, cars,motorcycles,audio,snowmobiles, and they all can get expensive in a hurry.  Back in my younger days I would drive sleds for some folks,drag racing, on ice,grass or asphalt. Nothing like driving a 300hp,$30k sled. Today your looking at 400-500hp sleds costing 50-60k. It never ends! 

Roger that. I can’t even imagine a sled with that much HP, but it would be cool to watch for sure. Holy cow, 400-600 HP sleds must sound like funny cars. Drag racing sleds, have to Google videos to watch later. Sounds exciting. I love watching the speed boats in the lakes in Michigan with their high HP V8 engines. I mean, these engines are built like one would build a high performance muscle car. Crazy loud too. 
When I started racing in Europe, it only took one lap at Hockenheim Germany for me to be addicted to racing. Put more money into the car than what I paid for it. But, man, that was a heck of a ride on the Autobahn. Yeah, all these diverse hobbies are fun for sure. Now, I am focusing on audio mainly, but, the car hobby will never go away either, hence, looking for another sports car. I guess old rockers never die, just fade away.