Spend enough to remove the weak links, then stop when a cable upgrade is hiding a bigger problem elsewhere. A fixed percentage can start a conversation, but your connections, lengths and system should set the budget.
Why the ten percent rule keeps failing
The old rule says a cable loom should cost about ten percent of the system. It sounds practical because it produces a number, but it counts the wrong things. A compact source and amplifier need two or three leads. A separated system with dual mono amplifiers, a subwoofer and a turntable needs many more. The same percentage cannot describe both.
It also ignores the cable you already own. A certified HDMI lead that works reliably does not need replacing just because the television was expensive. A long, unshielded subwoofer run that hums may deserve attention before the short interconnect between two premium components.
I would list every connection first: speakers, analogue sources, digital links, mains and any subwoofer or tonearm lead. Note the required length and connectors beside each one. That list prevents the common mistake of buying one impressive cable and leaving a physical fault elsewhere.
What sensible first upgrades cost now
Current list starting prices give a more useful frame than a universal ratio. The Oyaide EXPLORER V2 starts at $99 and provides 0.75sq 102SSC conductors in a flat, easy-to-route speaker cable. It suits shorter runs and smaller speakers where that conductor area is appropriate.
For an analogue link, the Jami RCA starts at $142. It uses shielded Oyaide ACROSS 750 V2 wire and rhodium-plated RCA connectors, and it is made to length. That is a sensible tier for replacing a loose or noisy free lead without pretending the interconnect costs more than it does.
On mains, the NEO by Oyaide AXIS-303 starts at $227, including 2.5sq 102SSC conductors and shielding, with equipment and wall-connector options. For USB, the Zonotone Grandio USB-2.0 starts at $339 and offers A, B and C combinations with separate data and power conductors and dual shielding.
Those are starting prices, not guaranteed totals. Length, a stereo pair and termination choices can move the figure substantially. Compare the configured price, not just the first number on the product page.
Where diminishing returns really begin
The first step should buy competence: the right impedance for digital, enough conductor area for speakers, sound shielding for analogue, safe mains connections and connectors that grip. Beyond that, you are paying for different conductor processes, more elaborate geometry, better hardware and finer mechanical construction.
Whether that refinement is audible depends on the whole chain. Speaker placement, room modes, an unsuitable cartridge load or a noisy ground connection can easily swamp it. Buying a dearer cable to avoid moving the speakers is usually an expensive way to leave the real problem intact.
The Zonotone 6NPS-3.5 Meister, from $522, illustrates the next tier with three 3.5sq hybrid cores, shielding and a braided jacket. It is a legitimate option for the right system, not a compulsory spending target for every CD player.
The order I would use
Fix faults before upgrades: intermittent plugs, hum, damaged insulation, wrong connector types and speaker cable that is visibly inadequate. Then address the longest or most exposed signal runs and the stock leads on the components you use most. Leave working, correctly specified cables alone until you can compare them at home.
Change one link at a time and keep the old cable. Use the same recordings and matched playback levels, and give yourself permission to hear no worthwhile difference. That is cheaper than replacing the whole loom on the strength of someone else's description.
For a system-specific shortlist, send the component list, required lengths and total budget through the custom order form. You can also compare the analogue interconnects, speaker cables and power cables ranges yourself. The advice should include where not to spend.