I've opined the rule of thumb to be about 15 % of your stereo budget should be spent on cables.
For me, there is a lot more to it than just sound quality. i.e. how do they look, will they lay flat without kinks on the ground and coil up nicely. Will the jacket last a long time without becoming stiff and cracking, etc. The sound quality almost doesn't matter so long as you have a solid connection, and it isn't so massive that it screws up the reactance of the signal path...
Some people will say, there is always something you can do besides buying new cables to improve your sound i.e. move speakers, acoustic treatments, etc. and this will be a better use of your $$$. But if you want an expensive cable just buy it and be happy. Don't worry about the improvements in sound so much, just enjoy the cable. because that's all that really matters at this point, wants. 'Cause all you need to make music is a set of toys-r-us bongos and/or a junky old guitar. Heh, I drum on my abs belly and chest sometimes.
The only real justification for one of those expensive cables is, IMO, you've followed the 15% rule and the rest of the equipment and room is that nice. That's another one of those qualitative qualifications, there. Nice. Not good sounding, or well built, or expensive (because budget is relative whether you are a homeless bum or billionaire). Nice.