Hmmm let's see.....I have no rule similar to Bishopwill. For example: I have a classic analog Kenwood tuner I paid $50 for. It sold for $200 24 years ago (which might be at least $500 today). I am using a pair of $250 cables (I paid $150). I love how it sounds. So considering what that Kenwood tuner might sell for new today, and the new price for the cables I spent 50% (but I actually spent 300% in real money).
The cable between preamp and power amp is also way out line based on price (high). However, since that cables influences everything running through my preamp (CD, Tape, Tuner, Turntable); if you allocate the cost between the 4 sources, what I spent per source component was a great deal considering how well it works.
The same goes for the power cord between the wall and the power conditioner when allocated to the whole system.