Hello,
hard to quantify what should be spent in either or other parts of a system. As some would say, it is rather the musicality reached altogether so if you have a weak point, you only hear its limits. I have spent no more than 700$ (audio project, kimber cables) in my cables but some are DIY or second hand. I would say around 20% and be sure not to have too long cables.
As I have different RCAs, it is fascinating to hear that it is a different sound if you exchange their place in the system… have you experienced this type of difference too?