It amazes me how some of these threads break down so much.
The OP has a budget and a goal. That goal does NOT involve building an AR-15, it involves buying an AR-15 rifle. Given that he's not subsequently stated anywhere in 130-plus comments so far that he wants to build one, let's get off that subject. Let's also get off the arguments over how much it does or does not cost anybody else to build any kind of AR-15. Not germane to the conversation.
Also, suggesting where the price SHOULD be on any given make/model is likewise not germane. This is a personal, subjective opinion. The market determines what these prices are, not us as individuals.
That said, there is some value in making recommendations for alternate make/models in the same price range as the OPs budget, provided it meets the OPs objectives AND there is some qualifying justification, preferably something other than a person's subjective personal opinion. Such might include discussions on proprietary components vs non-proprietary components when it comes to modifications/repairs. Another example might be availability. Or reviews which point out strengths/weaknesses.
The OP has, in 130-plus postings, indicated several times that he's focusing on the M&P 15 Sport and the Ruger AR 556 and given a few of his reasons. These have the basic options he wants and are within his stated budget; unless someone can actually point out where these two choices are proven Lemons of the gun world and somehow worthy of being used as nothing more than door stops, let's respect his choices and tailor our discussions accordingly.
There are more AR-15 platform guns out there than you can shake a pointed stick at and endlessly bickering over every possible choice OTHER than the OPs is endlessly circular. It adds to the total confusion of a person trying to enter into the AR-15 ownership world.
OP:
The S&W M&P 15 Sport is a fine rifle by my experience. I just bought one for my wife and it's as accurate and smoothly operating as any other rifle I've ever shot. So far, 200 rounds have been put through it flawlessly. There are several people I work with who also have the exact same rifle and have collectively put several thousand rounds through them with no problems and no complaints. In my opinion, it's a solidly built rifle made by a long-standing firearms company with a good reputation. It's a great starter rifle that I can't see disappointing you.
The M&P 15 Sport is a 5.56, which will also fire .223. The 1:9 rifling will stabilize the most commonly used .223 ammunition, which is 55 gr. Longer/heavier bullets may not stabilize well, but some people appear to have no problems with them. See this link:
http://www.gunsandammo.com/ammo/pair-barrel-twist-rates-ammo/undefined
The rifle comes with Magpul Folding MBUS rear sight, which is a folding sight like what you stated you desired. The front sight is a fixed, raised sight. If you want to mount a red dot later, you can do this; however, you will need a mount that will raise the sight up high enough to sight over the front sight. Based on my experience, a 37 mm mount will do this. I discovered this while mounting a Vortex Sparc II on my wife's rifle, which comes with a mounting system that will accommodate four different heights up to 40 mm.
I bought this rifle for $669 (before taxes). I'm perfectly happy with it, and my wife loves it. There are many very positive reviews of this rifle out there, like this one:
http://www.thetruthaboutguns.com/20...ger/-gun-review-smith-and-wesson-mp15-sport/
As for the Ruger 556, I don't have any experience with, nor do I know anybody at work that has one. Perhaps someone here can speak to that.