Percentage of cable cost

The cables do matter. Assigning an appropriate price is another matter. I am guessing that I have invested about 10-20% depending on the system they are in.
I can't imagine spending the kind of money I see spent. I think a more interesting question is how much is the most you have spent on an individual cable. In my case it's the speaker cables on my main system about $650 for entry level used Jena cable copper braid. To people outside the hobby I am sure that sounds insane. When I see speaker cable for ten times that -- well that's clearly over the top!

Out of curiosity, how many of you have ever done double-blind tests on cables? I suspect the results would be interesting.

Check the Stereophile website for a report on a blind cable test done in Minnesota. Good reading

I just read the test. Interesting, indeed. Not totally scientific, but the $8000 cable didn't beat the $3 cable by that much.

A few years ago, a noted economist did some double-blind wine tasting tests and found that even the so-called "experts" could often not distinguish between very expensive and very cheap wine.

Makes me wonder.

If I was still on the upgrade path 10% cost on cables is a safe bet. Before going higher optimize the room, a lot of what cables will add can be negated by reflections and standing waves. If unsure about major system changes in the near future stay on the conservative side as your needs may change. That being said once ALL system bottlenecks are removed or addressed that % can be higher and yield benefits as fruitful as any other big upgrade IMO. In my case of around 19% is also including power conditioning. My speakers, amps, and preamp will likely be the same for the next 10 years so this will be it for this system.