Why the disparity, is it worth it?

[Circuits]

Let me know when you find a $400 factory-complete AR.... not gonna happen, though $599 (50% more) has happened.

Most of the extra cost on high-end stuff is name recognition (they charge more because people will pay it), though some is innovation/exclusivity of proprietary designs and occasionally more attention to detail in and better quality base materials, and higher craftsmanship in the assembly, for those things that need to be assembled.

IMO: About the only truly justifiable premium cost for any rifle is a truly superb barrel, and then only if you're good enough behind the trigger to make it worthwhile, and are going to use equally superb ammunition. Good quality and unique design features of some furniture (premium buttstocks/grips/rails) might also be worthwhile, as would be a premium trigger, or at least work on the trigger.

Beyond that, the extra cost will just save you some time (over tuning and tweaking an otherwise stock AR yourself), and give your buddies some envy factor. Other than the barrel, everything else could be bought cheaply and worked into an equivalent for all the expensive bells and whistles, if you, the owner, have the time, tools and talent.

[mutt]

I have a Olympic Arms AR (considered buy some as a lower-end rifle) and a higher end Spike's Tactical with all the marketing points like CHF chrome-lined barrel, chrome-lined M16 BCG, DD FF Rail, MP tested, mil-spec blah, blah, blah and honestly I can't tell the difference between the two when actually using them. They both go bang every time I pull the trigger and both exhibit the same level of accuracy (I'm sure I the limiting factor in the equation).

I think Circuits is right that most of the cost is in the name and generally with the name you get more attention to detail. I think my Spike's rifle has a slightly better over-all finish. Think Vette vs Ferrari. Both are stupid fast but the Ferrari is generally going to be a work of art inside and out.

For a professional operator who's going to use his rifle hard I'm fairly certain all those little touches and upgrades offered by the higher-end brands are very meaningful. But to a normal guy like me, I'll never notice or need that difference for the most part.

[Great-Kazoo]

if you were to cover the mfg name on the lower of a factory AR. you would be hard pressed shooting wise to tell the difference. COM shots out to 2-300yards with a 16" bbl'd AR, is something almost every (hesse,vulcan the exceptions) mfg can do repeatedly without issues. Ammo, shooter, optics and mags are the variables in the equation.