Ah yes, I somehow missed the qualifying sentence "For simplicity I am leaving out the signal source".
The only other comment I would make is one that doesn’t help (sorry ;-) : The proposition assumes that all comparably-priced "same" components contribute equally to the sound quality of a system. In other words, if one allocates, say, 50% to a pair of loudspeakers, that two very different sounding speakers will both be responsible to the same degree for the assembled systems’ sound quality. It’s just not that simple. And then there is the fact that every speaker "type" (ESL, magnetic-planar, ribbon, dynamic) benefits more from one type amplifier than another, and different type amps come in at different prices to achieve comparable sound quality. If that makes any sense! If that assertion is accepted, the percent formula falls apart.
As has long been said, a chain is only as strong as its’ weakest link. The trick then is to assemble a chain with links as close in sound quality level to the others as possible. Price points are NOT necessarily indicative of sound quality. Two different $3,000 phono stages may not both be responsible for the same percentage of a systems’ quality. Assembling a balanced system is how and why good retailers make their 40 points! Now that many people (not having a good local hi-fi-dealer) are on their own, that's a challenge.