cost of speakers in relation to the rest of the system

kosst_amojan makes an excellent point: Designing and building good speakers is insanely difficult. In current dollars my guess is you'd need to spend $5,000 retail for something truly great, which would represent 50% of a decent system, however you can get thirty-year-old speakers that sound incredible for a fraction of that thanks to the fact that: There are minimal electronics inside speakers (so easily serviced/upgraded); speaker design/construction become mature technology by the eighties (thanks to a new design hegemony of aiming for a flat frequency response instead of trying to "voice" them, along with breakthroughs in driver construction); and, not least, audiophiles are collectively insane so the second-hand market is flooded. My point being: Buy a superb-sounding, twenty-year-old pair of Vandersteen 2Cs for $600 and all of a sudden the speaker to amp/source ratio goes crazy. There are no hard and fast rules to this.

These ratios are at best guidelines and will vary depending upon overall budget as well.  Ratios at $20-50k are generally going to be more speaker leaning in terms of total percentage than a $200k system.  There's also a general (incorrect) belief that good speakers can somehow fix shortcomings in front end and amp, which is in my experience far from the case.  Ultimately, unless the system builder is relatively new (and needs a helpful guide as a baseline) cost allocation is not a consideration that will be relevant; rather, it should be building the best system possible, and that is almost always a system where the relative performance of all components is similar since one cannot "fix" shortcomings in a poorer (relatively speaking) component and such component will then hamstring and limit the ultimate potential of the system.

@kosst "inexplicable urge to learn, build, test, and experiment."  +1.  I lack the confidence and skill to try to build anything electrical, even from a kit with clear instruction.  And I have a weird fear of electricity.  So, "mad respect," or whatever kids are saying on tv.

How often does, "I just like these speakers.  I know they aren't the best at X, but these make my ears happy" come into play?  I think some folks alluded to it.

I have an odd fascination with Infinity Primus 150s.  To me, they are one of the best bang for bucks I've run into.  I think they were about $100 a pair.  Bought four new pair years ago, maybe thought I'd use them in a 6.1 second multichannel (pretty sure at least one box has never even opened).  Still have them.  Still occasionally pull a pair out of the closet.  Still impressed.  Had a pair in a small system for several years and probably represented about 5-10% at the height of trickle-down equipment.  Just sounded more "right" to me than several others that cost several times more.

Let's not forget that many high end expensive speakers cost at least ten times what they cost to make. Ten times, think about it. I never heard this about electronics, this doesn't mean there aren't any. Those hypothetical $50k speakers are not that much high-fi as anyone would expect. No wonder that to get more or less realistic sound in a big room, especially with big orchestra playing, one has to go over $100k. For medium size rooms, as many said, forget about this level of realism, there is no need for $100k speakers. Yeah, $50k should be enough. I'd say, buying all new, that speakers' cost would be something like 30% - 35% in a $150k set-up.

There's the cost of making a thing, and then there's the R&D to develop the thing and bring it to market. Engineers don't work for free and the equipment they use isn't cheap. 
As for the electronics side of this hobby, having built a very decent amp, I can promise you that you're paying a crazy amount of money for the box they put the stuff inside of. If I bought the transistors in my F5 in bulk and matched them myself, they would have literally cost cents a piece. The caps in the power supply, the trim pots, and the non-inductive resistors in the feedback were about the only parts that cost more than $1. Buying in bulk the way a manufacturer does, they get much cheaper. 
Those crazy fancy cases that a lot of companies bolt the circuits into are at least half the cost of the product, probably much more than half in many cases. Do you think a Dan D'agastino amp chassis is cheap to make? It costs more than anything inside of it, I'm certain. Looks sell electronics even more than they do with speakers. At least a speaker might look weird for a functional reason. Electronics just look weird for the sake of looking weird so you'll pay more for it. Personally I'd like a stack of nondescript boxes that just do what they do very well and distract me as little as possible from listening. Spare me the glowing meters and flashing lights. If I want to know what my amp is doing, I'll bust out my thermometer and look. Yep... still hot as a toaster! Resume listening....