Is there a magic formula for spending on components in a system

It reminds me of house buying; the couple has an idea how much to spend on a house, how it will impact their life, etc. By the time the realtor is done, many have overbought, wrecked their budget, and will suffer relational stress as a result. So much for the plan.  

Go ahead an make your plan. Even select the picks for the system. Then, go hear them. Watch the entire plan shift and something utterly different end up in your place. Or, worse, run your plan remotely, and end up only marginally satisfied. It happens. Generic templates offer generic success, excellence at being average.  

As usual, almarg has great advice. The one aspect impinging on his analysis is that of the dramatic variability of quality of sound among components/wires/speakers. Make one wrong move on a system and you have severely compromised it. It's the reality of the performance game. 

Far more realistic to expect a nice result initially with a plan to tune/upgrade it as time goes on. 

I found the percentages outlined by Robert Harley in this book to be a useful starting point.

https://www.amazon.com/Complete-Guide-High-End-Audio/dp/0978649362/ref=sr_1_1?crid=DCQU4I651FUI&...

And I learned a lot of other useful information as well. 

In my experience:
Speakers 40%
Amplification 30%
Sources 30%

No money left for cables so use lamp cord.

Your experience is not mine 50% comes from the embeddings (mechanical,electrical,acoustical) only 50 % comes from the speakers, amplification and sources....

I am not surprized by your conclusion at all....Ignorance is bliss....

If you want to verify my saying takes any audio system, put it in a bad acoustical room, in a noisy electrical grid, with no controls of vibrations and resonance.... Good luck with the lamp cord....

Audio system at any price NEED TO BE EMBEDDED in these 3 environment.....

System synergy is hard to achieve but attainable at almost any $ figure - if I were a deep pockets retailer I would have say 5 differently priced systems with synergy I had verified in an untreated room. Many customers, maybe not quite most, would love to buy synergy without the cost of experimentation. My own journey to system synergy has been EXPENSIVE, I wish I'd found a shortcut sooner!

Anyways just my 3 cents.