Basically, if you intend to put your AR through heavy work, like through a carbine class, advanced carbine class, where you'll be throwing anywhere from 1000-3000 rounds within a span of 2-4 days, a few times a year, or even just once (YMMV)... buy the best material AR you can. You're already going to spend an arm and a leg for the course +ammo, you might as well have a gun that is more than likely to make it through the entire course. That intended purpose is worth the additional cost/pickiness into buying an AR with good reputation. @UGAARGUY condensed an adequate list using S&W as an example.
If you don't intend to put your AR through heavy work, no one here is really stopping you from getting the lower priced brands of nowadays such as DPMS, the Bushmasters, or even the underdog S&W MP15 Sports (this is an underrated weapon actually)... they will more than likely go bang for a few thousand rounds over the span of years. That said, I think for an entry AR, the MP15 Sport, for its price and critical features, is the best buy at $650-$700.
@MistWolf (tell me if I'm not understanding you correctly) is basically saying you shouldn't buy junk. Period. Whether or not you are going to the sandbox, or the sandbox comes to you, if you are type of buyer who's looking for an HD AR for the long haul, then by all means do not skimp on your purchase by settling for AR platforms that are sold with the, "Well you aren't going to war, this'll be fine" line. My further understanding is that given how the prices of quality ARs have now become competitive, there really is no reason to settle for an unproven brand that doesn't meet the most critical mil-spec requirements. Sure, mil-spec is "good enough for gov't work" or "lowest bid quality, not highest bid quality", but there's not a cheaper brand of AR that builds to even that minimum, so why settle for below the lowest federal bidder?
Personally, if you just want to plink around and will not rely on the weapon for SD/HD, you can buy any AR you want for very cheap and have a very fun time with it. It's very likely that you'll always be able to upgrade later should you choose to do so.
OP: The most popular brand ARs that enthusiasts here at THR will push you to such as Colt, BCM, Noveske (if you are lucky) LMT, Daniel Defense, Spikes (if you can wait 8 weeks) can be competitively shopped at the same price as the Colt LE6920 below or around $1200. On that list, the BCM will be your next lowest priced item at roughly $1050, with Spikes being at $860 for complete packages.
Palmetto State Armory is a newcomer but so far the reviews are favorable, and the pricing for what you get is fantastic, even better than the Spikes which used to be the best bargain @$800. PSA will get you an upper and lower for a combined cost at $650-$750. Aside from the MP15 Sport, these are two of the most attractive buys currently, at least when it comes to what you'll read about on THR.
Newbies to ARs... DON'T BE DISCOURAGED BY BUILDING YOUR OWN AR. It sounds scary, but it really isn't. Sorry about that.