it all just depends on what you plan to do with the rifle and what your needs are. if you just want a superlight carbine that alows you to hit man size targets out to 500+ yards... getting a $600 AR will do the job.
as your expectations get higher though, and you want to shoot something smaller and smaller only taking 1 or 2 shots vs 10-30 to hit your target... you will need something with higher tolerances

but that doesnt mean you have to buy a "built" gun from noveske or LWRC to get that accuracy...
you just have to build a rifle with quality products that WILL still cost you more.
most importain things... barrel, free float rail, and trigger. (trigger is open to debate cause i rock a stock milspec trigger... but it does help a lot to have a good one)
example... normal barrel will cost you around $100... the barrel in my AR is made by White Oak Armament... and it cost $400+.
thats the beauty of ARs though... you can build it how you want it