I think in the 8 speed KMC chains it's just finish and color. Price varies, but I think that's probably just the cost of the various treatments and what sells the most.

On their website, it doesn't appear they produce the gold color any more. So the gold chains you are finding are either left over from earlier production or that other bad word that will get us slightly off track.

The custom length chain is $5.00 for 118 links. If you want 119 links then there is a additional cost of $23.80 for a total of 28.80. I imagine it goes up more as you add lengths.

They don't really explain the differences between the different chains, but you can see both physical shape of plates and stuff is different. When you get up to the 11 and 12 speed chains, they do seem to give a little more info about them. But still, mostly might be just marketing hype. But for the cost of any of those chains, and knowing that you'll wear any of them out eventually, I'd just try out a different chain if I was dissatisfied with the life or color of the one I'm currently using.