This is a great thread, thanks OP! It seems like every time I upgrade a component I perceive a meaningful difference (good or bad!). In a haphazard way (which is fun, it’s a hobby after all!) I get a better overall system, through trial and error and by focusing on pieces recommended by reviewers, these forums, and design engineers I admire (such as Nelson Pass, John DeVore, Cees at Metrum Acoustics). Synergy, as said above, is key, but only discovered by trying so many permutations! And how do you know you’ve finally arrived?!
Point is that in doing it this way, the % spent on each category is in constant flux so for fun I calculated what it is at this moment. I buy mostly but not always used gear which can skew it quite a bit (for example I got a great deal on a wyred4sound stp-se stage 2 upgraded preamp for a 3rd of its new retail), so I calculated it both ways based on new price as well as what I paid:
New pricing: speakers-37%, amplification-39%, source-19%, cables-5%
Used pricing: speakers-29%, amplification-34%, source-29%, cables-8%
Accounting for used pricing brings the percentages closer to parity, not sure if that is interesting or not, but there it is!
Ron