Hello,
In my opinion, the most important cables are those that are the very first in the system, especially if you listen to an analog source. Good cables between the tonearm and phonostage can make a critical difference. How much to spend? I suppose it depends on how much disposable income you have available. After all, everything is relative.
That said, other cables make less of a difference, although the differences can be heard. Cables in the chain before the speakers are very important, of course, but there is a diminishing return. Speaker cables are also important, but they may be the least important ones. By the way, I own a pair of Tesla treated 6N copper speaker cables that would sell for thousands of dollars, and they are better. How much better? I suppose that would depend on all the other components and the expectations of the listener. They are very good, though. Would I pay thousands of dollars for speaker cables given my current finances? No, but mine were a gift from a manufacturer friend. Anyway, if someone has an extra $15,000, and he hears a difference that justifies the expenditure, it is his money to spend. I see the entire issue as one that is wholly dependent on ones lot in life, and his particular priorities relating to our hobby. No one should begrudge him for that. Just count your blessings for the system you have, and be happy that an upgrade path will always exist for when our ships come in. ;)
Regards,
mosin