I haven't seen such a bargain on Oly or DPMS. Wouldn't mind to, just haven't run across it. However, from what I've learned:
A1 = Carry Handle
A2 = Carry Handle w/ windage
A3 = Flat top
If this is correct, I am in the market for an A3.
That's essentially correct for the different commercial designations. In the military A3 means something else, where A4 would be the correct term for a flat top rifle (with 20" gov profile barrel and 3 rnd burst).
There are two potential problems with going with lower end ARs. The first is the possibility of immediate problems- stuff out of spec, stuff that's broken, etc. Most of the modern makers do not suffer from these problems (the exception being Hesse/Vulcan). The second problem is more insidious; the usage of substandard parts that are dimensionally proper but materially suspect. This means that they work fine until they fail, and that they fail sooner than "top shelf" parts.
Now, how huge of a problem this is, I dunno. But listen to the guys who run carbine and rifle courses. They are the ones who get to see all of the rifles put through their paces, and NOT in a "well I went out to the range and shot 20 rounds" manner. Most of them have pretty strong opinions about what works and what doesn't, and they're usually flamed by the people who spent $600 on a rifle that is "just as good" as what the trainer is advocating.
Now, if your rifle is a range toy for gun games or just plain old fun, a bargain AR is probably no big deal. If a part breaks, big deal. Use the $600 you saved off of a Colt rifle to buy replacement parts and press on. However, if it is a 'serious use' gun, part failures can be fatal.
As to what the failure rate is for "cheap" parts as opposed to "good" parts, I have NO idea. So, it's hard to know exactly what benefit you're getting for your money. The guys who do it for a living, though, claim that it is worth it if you want your rifle to really run in an unforgiving heavy-use environment.
Mike