??? How Much Would You Spend ???

The bottom line is that what you can afford will determine how much detail,headroom and depth of music you ears will hear...      

And you can listen to any sound system at a Retailers but until you get it into your sound room of playing you really will not know  how it sounds with your room acoustics...

I ask my wife every time I make a change do you hear this or that and has No Clue what it is I listen for or hear...

I know this last purchase of the Reflection is at the Peak of what I I will ever hear from my system ...No Regrets and have to wait for wife to go out so I can enjoy my music..LOL

If I won the powerball I would buy those million dollar speakers.still trying to win until then I luv what I have.enjoy the music

I don't think a cost percentage is the best way to approach building a system.  As others have said above, you do need to match the amp to the speakers, so it may be more or less than 50% of their cost, especially if you buy used.  My amps' new value was twice that of the speakers, but they are still in use after over 30 years, and they came with a lifetime warranty.  If you don't want to upgrade in the future, you'd want to spend extra to get durable equipment.  I don't know if there are any amp manufacturers now that offer a lifetime warranty.

I do agree with those who prioritize a good preamp.  I bought a top-of-the-line Classé Audio DR-7 in the late '80s and have never regretted it.  Aside from its sound quality and durability (still works great), it has great features like a knob for reverse stereo/left channel/right channel/mono.  Having such useful features (at least useful for me) as well as top-quality sound really adds to the enjoyment and maintenance of the system.  On a newer preamp, a remote volume control would be a great thing to have.

Here's a problem I don't think has been touched upon...

Common guidance is to buy speakers first, then amp / preamp, etc. 

But - when you picked out your speakers... what amp was driving them? How big was the listening room? Was the setup and room in any way comparable to the space you will be using in your home..?

Don't mean to muddy the waters or be purposely obtuse. Just saying that there really shouldn't be any 'hard and fast rules' when choosing your gear. Even how much to spend, by % per item or total budget.

I guess my advice (for what it's worth) after having owned various combinations of somewhat decent gear for the past dozen years or so is this:

 - Choose what appeals to you most. And base that appeal on a combination of your senses. Find a 'happy medium' where the gear appeals to your ears (sound quality), your eyes (looks impressive / appealing to your taste), your touch (functions the way you want it to, and has the functions that you are looking for) and also appeals to your wallet (doesn't diminish your happiness  / satisfaction factor because you spent more than your own common sense dictates).

I think too many people chase 1 or 2 of the parameters above - especially the sound and spend ones - in a misguided effort to either impress others or convince themselves that they have "the best". I think that it bears repeating that, other than other 'audiophiles'... the "impression factor" hits a ceiling with the average person. They are going to think a $1K system 'sounds great'  - and they are going to think a $20K system 'sounds great, too'... besides, chasing allocades is a terrible reason to be in this gig, anyway.

Myself, I am partial to 'vintage' gear (60s - 70s) and my common sense looks for gear that needs a little work but has great potential. My 'pride in ownership' (if you want to use that term) comes from telling those impressed with my system how little money - and how much of my own restoration labor, etc. - I put into my system. I know it's still bragging - but at least it's not a simple 'look how much $$$ I have, and have spent on this setup..."

Anyway, maybe I should answer (at least partially) the OPs question after this longwinded rant...

In my experience, speakers make the absolute biggest difference in how your system will ultimately sound. And, to that end. I found the best sound (to my ears) by trying to place the largest displacement speakers that would reasonably fit in the space allotted (without looking cartoonishly huge and out of place). A bit challenging in smaller rooms, but IMO worth the effort in the long run.

I will spend all your money if you let me.