I agree with many of the comments, particularly "what is expensive?" In the context of my modest system, a $50000+ cable loom of Crystal Cable Dreamline or Nordost Odin say, would be insane, but in a $500000 system, maybe not. It is quite another subject, whether a $500000 system is insane, I think it is, but I am not a Hedge Fund executive, with to much money and not enough to spend it on.
There is no question in my mind, that cables make a difference, sometimes only a difference, not an improvement and that takes time to judge. That is all cables, I was initially sceptical about Power Cables, now I think they can make a massive difference. So I am prepared to pay in the region of $500 to a 1000, retail for cables, preferably a lot less second hand. That to me, my wallet my system, is the comfort zone. It will be different for others and that is for them to decide, it's their money.
Two big areas of advantage in the cable area as opposed to other equipment. Firstly, cost of entry is low, so there are many one man and his dog outfits out there, selling direct, with minimal advertising and non fancy packaging, that I have found are better value for money than some of the bigger outfits. In no particular order, WyWires, Morrow, Silnote and Sablon Audio, come to mind.
Secondly, second hand prices are often very good, particularly for a product with a potential very long life. No transistors, Styli, speaker cones or tubes to be replaced.
So to me, sensible purchase of wires, along with other accessories, vibration devices, power supplies etc, can bring a bigger bang for your buck than replacing Speakers, Amps or sources. They can make a good system great and give long term satisfaction, with no need for further tinkering. That is where I believe I am now, I'd better be, retirement means I can'nt go on indulging in new big purchases.