As much as needed until you reach the point when you no longer hear any meaningful difference going higher.
And when you buy used you can sometimes get lucky and have $2.5k cables for $600.
It also depends on the complexity of your system. If you have one source component and integrated amp - that's one set-up, and if you have three source components, separate transport and dac, separate phono stage, preamp and power monoblocks - that's something different.
I could live with one of my $500 interconnects on $1200 phono stage but what do you know - I have $1500 cables on it and the $500 one is in the closet for rare cd player use, which is also in the closet. My speaker cables cost new as much as the speakers, and power cord for the integrated amp more than the amp. All bought used except two pairs of interconnects. Now all my active components sound as good as they can, I believe. Soon it will be time to build a completely new system.
However, if you want the simplest answer to your question, I would say 20% new, generally speaking. Tonearm cable is very important if you think about playing records or play them already.
Each cable is a component and all of them much more so.
