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.