??? How Much Would You Spend ???

Totally agree with @rick_n !  Preamps are crucial in my experience.

The little fosi audio mono is about 300 watts with great reviews parts express has a sale on them now they are great for small rooms you could get a chit tube preamp it sounds great.i have high end michi krell mcintosh bryston pass parasound cary and others.these little fosi mono amps are a bargain.mcintosh marantz denon audio research and other major brands have gone class d as well.enjoy the hunt and music

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.