how much should you spend on cable?

My opininon is that you need cables with an acceptable level of clarity and a tonal balance that compliments the equipment being connected (and your musical taste). Speaker cables should be a minimum of 12 gauge (no higher, unless the length is very short). I would not allocate any % of a total system cost to achieve this but instead would try and spend as little as possible, w/o severely compromising the overall sound. Be wary of the law of diminishing returns and the laws of physics. Expensive is not always better if it doesn't interact well with your other gear. Spending a little more to get a right of return or good advice is usually worth it.

FWIW, I started by spending as little as possible on a recent system build ($2150 1st audiophile system for a friend) - cables came out to 15% of the cost, and performance improved markedly with every step. Most of my systems have wound up with 10-20% in speaker/IC cables.

I think I'd be scratching my head at > 25%, wondering what went wrong.

Anyone who tells you there isn't a forumla is correct, the numbers I gave are just from my personal experience.

I don't know that you can put a price on cables. If you find cables that provide you with good system synergy and they cost .01% of the rest of your system, does it really matter? I don't think so. On the other hand, if I had a 5000 system, I wouldn't be looking at Kimber Select Black Pearls either.