Percentage of cable cost

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.

I do not see any valuein chosing cables based on a %. All that assures is you spend a certain amount of money, nothing else. Very good wires do not have to cost a fortune. Makes no sense really.

the proportion spent on cable should be related to affect cables have on the sound of your stereo system.

I have never gone by the "percentage" rule with cables. I think that an expensive cable is worth it when it's the cheapest way to improve your system. IMO, a box or speaker upgrade should sonically exceed the sonic contribution of a cable upgrade. But when your system becomes expensive to the point where better cables offer a significant sonic upgrade for much less then a black box upgrade, cables are the way to go. Cables are the icing on the cake, but you need to have the cake first.

I feel cables can make a significant difference, and yes, this has included double blind tests in my system. Current cabling cost varies from 10% - 15%. I do find that used cables hold their value extremely well, so ultimately have spent very little on cables.