What makes a DAC so expensive?

A dac is the most difficult thing to buy for me in an audio system....

And it must not exist.... When nothing suggest to you his existence this is the good one....

When it comes to the DAC: Is distortion the only word (or the most important...) in transparency AND full realization of the recorded sonic imagery? 

I have been lead to believe that some DACs handle detail and soundstage differently than others and I'm wondering if these features (of subtle detail and soundstage) are fully reflected in distortion figures. 

This is to say, FOR EXAMPLE, what kinds of (general) differences might be reflected in two different DACs, with the following THD+N measurements:

• 0.0003%
  

• 0.003
  

Or what else am I missing, for purposes of discernment, other than the in-my-ear experience of evaluation...

i reach and create a very great S.Q. in the last 2 years: dynamic, clarity all across frequencies,details and more, soundstage in all the room, imaging with depth in near listening and in regular position, 3-d holography, name it...

All that with my controls of the mechanical embeddings, and all this time, i never think one second about my DAC.... It does not exist at all.... And all this time this thing that  never has signal his existence to me, never did’nt hold me back ever, in each and every one step toward better S.Q.....

This is what i call a DAC....i will never look back....

What makes a BMW 230 cost twice as much as a Toyota Corolla?  After all, they are about the same size - the Corolla is probably a little bigger.  What makes Veuve Clicquot champagne cost four times as much as Freixenet?  After all they both come in the same size bottles with very similar corks.  Just in case you think that this is purely snob appeal, I have driven both cars (quite recently) and drunk both champagnes, and I can tell you from personal experience that they do not drive/taste the same at all.  I have a $25 DAC from Ensinin [?] and a $99 DAC from Schiit and even with my hearing loss and untrained ears I can tell you  from personal experience that they do not sound the same.  

Engineering is expensive. Parts are expensive. Manufacturing is expensive. You can go the cheap route or the bespoke route. If you really want to know why good DACs are expensive, read the reviews!