Bit late to the thread, but have spend countless hours reading and researching this very same thing over the past 6 months, so I'll offer up what I can.
For starters, I wouldnt call any of them cheap junk. They meet mil spec, and do as they were designed. However, some cost more because they have more features to offer, or better accessories, or better name (colt).
First you need to decide weather you want piston operated, or direct impingement (DI). All other things being equal, DI guns are cheaper. They are a little dirtier, fowl the bolt a little more but are alot cheaper. Military guns are DI FWIW. Piston operation is not a military thing, therefore no mil spec to that. And have read that not all piston components between MFG's are interchangeable. Whereas Anything on Any DI mil-spec gun is interchangeable.
Next, decide what length system you want. There are 3 different types, Carbine, Mid-lenght, and rifle. This has to do with the lenght of the gas tube, (where the port in the barrel is). IIRC, Carbine is 7", Mid is 9" and rifle is 11". This effects how the gun "feels". If you pull off hot gasses at the 7" point, they are under alot more pressure, as is the case you are trying to eject than at the 11" point. Many folks may not even notice a differnce, but the result is a carbine feels more "violent" in its action and cycling than a rifle length. A mid length is an in-between.
Third, decide what barrel length you want. The reason for carbine length gas systems is for shorter barrels. There needs to be enough barrel forward of the gas port for the gun to function. That cant be done on a short barrel and rifle length system. Barrels have to be at least 16". If you have future plans that may include changing flash suppressors, get one that actually has a 16" barrel. Some of them have a 14.5" barrel from the factory, and they have to pin (otherwise make permanent) the flash suppressor to get the 16" length and you cannot change it later.
Fourth thing to consider is the barrel. There is ALOT that goes into barrel, and lots of acronyms and hard to get it straight. Like M4 barrel, or A2 barrel. You can google something like "ar15 barrel profile differences" and see tons of pics. But an AR barrel isnt just a simple barrel like a rifle. IT is a complex piece that is thin is some spots, thicker in others, steps and shoulders for accessories like a grenade launcher (not that we need to worry). But probably more important than profile is material. There are basically 3. 4140, 4150, and Stainless. Then a lining. You can have a barrel that isnt lined, chrome lined, or nitrided. The nitrided is claimed to be harder and better than chrome, and more accurate since it isnt really a coating, rather a hardening treatment that dont change the dimensions of the barrel/lands. Chrome 4150 is mil spec. 4150 is better than 4140.
Now there is alot of back and forth on barrel material and lining. Military uses 4150 because it is a little stronger, but when really hot, it is quite a bit stronger. But who we kidding, Plain 4140 has worked for YEARS for barrels. So this is one of them things that the "cheap" stuff usually has 4140 non-lined barrels. Or even 4150 but unlined. Where the more expensive stuff is lined or nitrided 4150.
Beyond all that is just accessories. And thats where more of the A2 and M4 jargon comes in. Fixed or collapsable stock, carry handle or flat top, etc. Its just accessories and personal preference at this point.
There are probably 10+ guns in the $650 or less range that are GOOD guns. They lack some features that make the whole gun mil spec to shave some cost. Some dont have a dust cover, some dont have forward assist, no chrome lining, etc. But they are still excellent and reliable guns.
Delton sport
S&w MP Sport
Rugar AR556
Core-15 scout
DPMS has a few models in this price range
Bushmaster
All the lower cost guns will be DI operation and most will be carbine length gas system.
There are TONS of manufactures out there making good quality AR-15's. If you have a specific question about one in general, search or look it up or ask here. But its really hard to go wrong with any off-the-shelf AR. They all make good brands and different configurations in may different price ranges. You just have to figure out what features are important to you, and what you want to spend, and see if the two Jive.
And as mentioned, you can always do a custom build to get exactly what you want. I have a DPMS and a Core 15, but just bought a few stripped anderson lowers. You can get so much more for your money building, but the re-sale is tough on a custom gun. But if you buy an off the shelf gun, and decide you want something different, they dont loose their value much. I see alot for sale on arms list with "only 100rnds shot" and they are asking more than I can buy new for.
