How good does my system have to be to realize an improvement with cables?

I've heard the difference power cables and interconnect cables in a highly resolving system so that's not the question.  $200 for cables  on a $500 DAC doesn't make sense to me.  Wouldn't that $200 be better spent on a better component?  At what point is an expensive cable the best choice?  Spending $500 on a $15,000 DAC does make sense as the transparency and resolution of the component could be better realized with a better signal / power source.

My question is what's the  percentage of the overall component's cost should be the cabling and at what point should it even be considered?

Or put a different way How would you know if your cables are your weakest link?