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.