In my opinion based on my limited experience, the modularity of the platform pretty much upsets the old paradigm about cost vs. quality vs. accuracy vs. etc. etc. etc. This is a totally different universe from the bolt-action world.
The way I see it, if you like your trigger, the barrel is accurate, the BCG is durable and reliable, and minor parts don't break then you've got yourself a perfectly useable rifle.
A crappy magazine can make a $2000 Daniel Defense AR look like a piece-o-schidt.
If I have a brand new BCA and a brand new DD rifle in comparable configuration and begin swapping parts between them, at what point does the DD become a turd? At what point does the BCA become a "good rifle?"
Considering the purposes and functions of various components or parts of an AR, I believe that it's the BCG, the barrel, and the trigger that make the gun. And it all depends on a good magazine to run right. Seems pretty simple to me.
