I’m in a similar camp as Jim and Famavolat. The short answer about cables is…it depends.
I bought a used pair of Magnepan’s for about $3000. I upgraded my $500 used pair of MIT speaker cables for a (new) $1500ish pair of MG Audio Design cables. The change in tonality, clarity and top end really blew me away. So in this specific instance…50% was the number for me.
What I am not sure about is power cables. My amp costs about $10,000. I am using a $250 PS Audio cable and I think it’s great…compared to the stock cable. I cannot justify the same 50% ratio, and spending $5000 for a power cable for my amp…unless, of course, I tried one and it blew my socks off…
In the power cables scenario, I also did the A/B test with my better half as the listener…with me switching cables behind my rig. every time she picked the PS Audio cable. more refined bottom end.
but, to the question at hand, I would suggest the listeners preference in presentation of the sound is an important factor to consider when choosing RCA interconnects. back to the objective. what are you trying to accomplish?
if you tend to like bright or crispy presentation, consider a cable with high silver content perhaps? or a bit warmer sound, with higher copper content in the cable design.
acoustic zen makes a nice set of RCA’s in each cable type - copper or silver. I’ve tried each type (silver and copper) of the acoustic zen brand, and could easily distinguish the flavour of each cable. smooth and warm vs. precise, dry and crispy.
so…it depends…on what flavour condiment you prefer (as mentioned above), and the benefits a more expensive cable adds to your particular system.
I would think a $300-400 RCA set of cables (or less on used market) would bring a nice flavour to your setup.
my 2 cents. (Canadian cents) 
ps. one more thing…if your speaker cables are from your basement bin, perhaps your money would be best spent on upgrading speaker cables first.