What makes an expensive speaker expensive

"Speaking of crossover parts, it is easy to take an inexpensive low and mid grade speaker to a much higher level by just replacing the caps, resistors, and inductors to higher quality ones"
While I agree 100% with audioman2015,  I want to caution anyone about replacing Inductors.... Each inductor, yes has a inductance rating in henry or millihenry, they also have a resistance rating... For those that don't know,  a speaker may be crossed at 6db per octave at 2500hz,  but when you change the inductor,  if you do not compensate for the impedance change in the inductor itself,  you have effectively changed the crossover slope or point or both.  So for those that want upgrade,  that's fine, but unless you are capable of these measurements, I would recommend against inductor changes..... ESPECIALLY going from a cheap steel, iron or ferrite core to an air core... these resistive values can be dramatically different. I hope that this is helpful. 
Tim

timlub - I completely agree!!  Resistances need to be compensated for the inductors.  There are plenty of crossover calculators online and measurements are always useful if changing the inductors.  If there is a drastic change in the crossover point, it could damage the drivers.  

A friend of mine has a pair of some vintage Sonus Faber's and the tweeter was using an electrolytic cap for filtering!!!!  We changed it out to a low end and inexpensive  film ClairtyCap (~$5 each) and his eyes nearly popped out of his head and realized the full capability of the tweeters.  It was like a switch was finally turned on!

Hi audioman2015, yep, recently recapped some old Kef's for a buddy,  same scenario,  he fell over.
Tim

More times yhen not the cabinet can cost more then the drivers
And Xover which it should not. Companies like Harbeths 
Not much to the cabinet, or that expensive of drivers.they have a excellent patented midrange material  ,that being said the xover vs 
The big names that use premium parts like Mundorf silver gold oil 
And top resiryors,inductors top quality wiring  harbeths uses nothing even close but charges a high premium vs actual build cost like a garage build,with quality veneer. Open baffles if using Top drivers ard a Superb bsrgsin, as is new Martin logan,Maggi
Even Electric with engineer Jones forgot first name builds great
Speaker for thd money .most big $$ speakers are a hyped up scam
I have worked with loudspeaker design and cost they add all $$ Per hour in what they want maybe $200 a hour in R &D time plus 5x in cost for material s thrn overhead. There is the price  
Look at Magico, Wilson Radho, YG Acoustics. Sure they sound excellent  $100 -to $200 k for s pair of speakers give me a break .
Evrn 50k. $20'k should bd absolute max plus 20% off.
TheRe are a lot of quality speakers even under $3 listen first z,!😆

Replacing x/o parts for higher quality ones is a high-return-for-money-spent tactic. But there’s a further step that can be taken: Danny Richie of GR Research, well known for his x/o designs, not only makes cross-overs with improved parts, but offers x/o corrections for basically well designed but budget-compromised speakers such as the new line from Elac, to improve frequency and waterfall plot (time domain)performance. The simple addition of one part corrects the falling high frequency output of the Elac tweeter, and another simple change to the x/o frequency improves the speaker’s midrange. A little bit of money yields large improvements!