If cables is what you’re working on. If all your other major components are pretty stable, I find price and performance has a direct correlation. In general cables that cost more sounds better even in a modest system you can hear it. I would take the recommendation of of some of others in this community with experience in the more affordable ones. It also depends on how your system is balanced. Cables can sometimes emphasize certain frequencies, so an overly bright system might be better balanced with duller sounding cables. You have to find the ones that compliments your particular system. You might have something in mind already and I find something like the Cable Company let you try many cables before you buy. You might want to take advantage of that.
When BHK300’s entered my system with Audio Research LS28 preamp & Focal Sopra No. 2 loudspeakers decided to upgrade 3 pairs of Monster Cables (speaker, preamp to monoblocks, preamp to Sony SCD-XA5400ES CD/SACD player). Over a 5 week period compared Wireworld Eclipse 8 with Tara Labs Air Evolution in my system. Convinced going in that, though much less expensive, the Wireworld would be, if not as quite as good, at least close enough to choose over the Tara Labs. Didn’t turn out that way. Not trusting a possible bias in favor of more costly Tara’s, my wife, how doesn’t know Wireworld from Wally World, spent a considerable amount of time listening to a wide variety of music over several sessions, during which time I switched cables back & forth. She never knew which cable was being used & still has no idea how much any of them cost (and, please help me keep it that way). Dozens of switches, she preferred the Tara EVERY time.
Moral of the story: you really have to listen for yourself, in your system, over an extended period of time. Your system will tell you how much to spend.
OK just for fun if I’d build a $20k system from scratch I would also budget about 10% on cables. This is the breakdown that feels about right to me:
Total 20K
Speakers 10k
Electronics 8K:
- Streamer 2k
- Pre 2k
- Power 2k
- Powerplant 2k
Cables 2k
Weirdly a $20k budget now feels tight…
I’m running speaker and interconnect cables, Iconoclasts, that were more than my current in use speakers I purchased used. Significant different vs. good quality Belden cables used prior with the same setup. FWIW, I have 3 sets of stand mounts that get rotated in our out depending on mood or need to change it up. They all sound great but each are different.
I have a small space and they just don’t make small 2-ways that cost very much so it skews things a bit but what I have certainly resolves the differences in speaker cables and interconnects.
Try before you buy, the price is often not indicative of value or performance in my experience.
I often hear spend the most on speakers but I counter that with it depends on the space you need to fill with music. I can’t physically accommodate floor standers. Awesome stand mounts are far less $, add a couple very good subs and rock n’ roll.
I usually buy used when possible but if you go by new prices my modest system, which includes the optimized NUC and hard drive dock I use for streaming, would cost around $15K and I currently have maybe $2800 - $3000 in cabling/power cords which comes out to around 18 - 20%.
If you know the provenance & trust the source, used is a good way to go. Purchased an 8 foot pair of Tara Labs RSC Air Evolution speaker cables, which retail for $4,200, for $1,700 from an authorized dealer that I’ve been dealing with for many years, who originally sold them new to a client & a year later took them in trade when he upgraded to the Muse. 60% off & fully broken in. Win-Win.
Yeah, not long ago I would have thought this absurd and impossible.
For better or worse, over the past year, I’ve learned enough to feel the same way.
Started with a splurge on a $600 Thorens turntable. 
You should do everything you can to not change from how you are now. Can’t hear a difference, how absolutely wonderful for you! I always say how sad I was when I could hear the difference between a Platinum and Silver cable from Wireworld. I am super offended that I can hear a difference. I wish it weren’t so.
So, don’t let well meaning friends bring expensive cables to your house. Don’t take advantage of return policies. Just say no.
My two cables arrived today. They were a steal at $2400 each. God.
(I bought them due to the 20% off sale, one for my P20 and one for my DSD. People here said they could not believe what a wonderful difference they made. I am looking forward to Wonderful)
I’m in a similar camp as Jim and Famavolat. The short answer about cables is…it depends.
I bought a used pair of Magnepan’s for about $3000. I upgraded my $500 used pair of MIT speaker cables for a (new) $1500ish pair of MG Audio Design cables. The change in tonality, clarity and top end really blew me away. So in this specific instance…50% was the number for me.
What I am not sure about is power cables. My amp costs about $10,000. I am using a $250 PS Audio cable and I think it’s great…compared to the stock cable. I cannot justify the same 50% ratio, and spending $5000 for a power cable for my amp…unless, of course, I tried one and it blew my socks off…
In the power cables scenario, I also did the A/B test with my better half as the listener…with me switching cables behind my rig. every time she picked the PS Audio cable. more refined bottom end.
but, to the question at hand, I would suggest the listeners preference in presentation of the sound is an important factor to consider when choosing RCA interconnects. back to the objective. what are you trying to accomplish?
if you tend to like bright or crispy presentation, consider a cable with high silver content perhaps? or a bit warmer sound, with higher copper content in the cable design.
acoustic zen makes a nice set of RCA’s in each cable type - copper or silver. I’ve tried each type (silver and copper) of the acoustic zen brand, and could easily distinguish the flavour of each cable. smooth and warm vs. precise, dry and crispy.
so…it depends…on what flavour condiment you prefer (as mentioned above), and the benefits a more expensive cable adds to your particular system.
I would think a $300-400 RCA set of cables (or less on used market) would bring a nice flavour to your setup.
my 2 cents. (Canadian cents) 
ps. one more thing…if your speaker cables are from your basement bin, perhaps your money would be best spent on upgrading speaker cables first.
To show the system, whatever system, in the best possible light – to the extent that I can detect that light.
Power cables should cost 1.5x what you are hooking them up to.
Speaker cables should cost 3x the cost of the speakers.
Interconnects should cost $7000 per month and be switched out at the end of each month.
Jeebus - that’s $20k? Some price shopping is in order. : )
But anyway - agree that there is no rule of thumb.
I would approach it as most of us do - try as many cables as possible for free, for a reasonable amount of money or for a “rental fee”, and Hear what you Think.
I’d say 20% and more is no bad rule, others might tend to 10%. But to choose the right one is more important… that’s no problem if you have the chance to compare in your setup.
The more revealing the setup, the higher the percentage should be. My 2 ct.
i knew someone would get out the calculator 
I assure you, I did not. However I was vaguely familiar with those prices. Darren just bought 30.2’s, and we’re generally familiar with most of the rest. Still have no idea what the total would be.
Plus it is not about the Total.
I think the important questions are:
- Who is Larry?
- Do you even like Larry?
Of course there are also a couple of rules about the Expensive Cable Club:
-
You don’t talk about the Expensive Cable Club
-
You don’t talk about the Expensive Cable Club
Of course I am kidding a bit but I do tend to budget a lot for cables and power stuff knowing I will use them into the future even if my system changes just so I have a good base to build off of and don’t have to go replacing stuff later.
Yeah…sometimes I feel like I’m listening to money and not music. Not a good thing…
That is a good thing to bear in mind - A For Realz Rule of Thumb - if you’re listening to Money rather than Music…fugettaboutit.
Larry is Very Important in the Equation. Perhaps more important than any other aspect.
Not kidding. 
I was just playin’!