How much does power amplifier really matter comparing to other hi-fi components?

 I've never built any system in nearly thirty years with a set percentage of budget for any particular element in chain. I've always started out with amp/loudspeaker as package deal, whatever loudspeaker I started out with required certain type of amplification, I purchase the best I can afford at this point. Rest of system follows from here.
Certainly, there is a certain percentage attached to each element at any single moment in time, just don't pay attention as I'm building system. I can only say that every single link in chain is important, when weak link discovered, 100% of my efforts and budget goes toward eliminating that weak link.

I'll preface my statement with, I don't believe in absolutes in audio, because all rooms/systems are different. That being said, I recently changed from my Jeff Rowland 625 S2 amplifier to a pair of Jeff Rowland 735 mono-amps. My dealer(Albert at Precision Audio and Video) has stated for years that, "I never really heard my speakers and he recommended upgrading my amplifier." To say the amplifiers made a difference would be an understatement. My wife never comments when I change components. Her comments, "voices and instruments have more depth." I used REL G1 mkII subs with the JR 625 S2. I sold my REL G1 mkII subs. I prefer the sound of my system with the 735s, without the G1s compare to the sound of my system with the 625 S2 amplifier and G1s. So, in my case changing amplifiers made a significant difference.

I spent 50% on speakers....  60 % on amp , 50 % on sources....

^How much was spent on maths studies?:-)

Everything adds to the sound quality, but some things count more than others.  The speakers that you buy set the tone for the rest of the system.  Expensive, very revealing speakers will be more sensitive to sources, amps, etc.  "warmer," less detailed speakers will be more forgiving of other problems, so you will be able to spend less on the other components.  That being said, I listen exclusively to digital these days, and with a very good DAC/power amp combo, you have the foundation for a system that will allow you to expand the quality of the speakers without having to upgrade everything.  Remember that more than anything else, speakers play the biggest role in shaping the sound that you hear, while you don't have to shell out big bucks for an amp to get great sound out of them.  Get the system set up for your budget and then upgrade things like cables, power conditioners, room treatments and the like.