I understand your thought process, assuming the end user has the basic expertise to do the job. But for those who don't or who don't have the time, I would say 10 to 15% maximum would be about right.
I'll give my recommendation here...so backspace now if you don't care.

I've been throwing money at this hobby for over 25 years now. If a listener decides to buy their cables, I'd recommend they opt to go with someone who has a smaller operation and a passion for the craft (along with a great reputation) rather than a larger shop with a passion for mark-up. That's how I found Grover Huffman. His rep and references (including Steve Hoffman) are top notch. Even though it was all on reputation, I took a chance starting with interconnects and digital cables, then speaker cables...and I've never looked back.
All of my cables (except for USB, HDMI, and power cables) now come from Grover. I've auditioned cables in my system that cost many times as much and heard a LOT less.
People will tell you that wires are wires, electrons are electrons, and cables make little if any difference. Well...if it tells you anything, I find myself wandering down to my basement to listen to music more now than I have anytime in the last 10 years. I attribute that to what I'm able to hear now as much as anything else.
BUT...if you're happy with what you have, then don't change a thing. I've been a musician for more than 35 years and knew that there was more to hear that I wasn't getting from my system. His cables have helped me get it out. I don't have to buy cables or equipment any more...I'm there.
(edited 2011-03-04 for bad grammar)