Like I said in my last post, I agree with you. You couldn't build one for $6,000 for that matter, because if it is fully mil spec, it will have to be a transferrable Class 3 firearm.
You can, however, put together a rifle that is as close as you can legally get, without going NFA, in the $600 range using a PSA upper and BCG.
This is why I put "mil spec" or 'mil-spec' in quotes, because I am trying to avoid the super duper nit pickers who will say that because it isn't a machine gun with a 14.5" barrel it isn't a mil spec M4.
Also, "$600 range" =/= $600
You cannot build a "mil spec" AR15 style rifle, new, right now, for $600.
You can argue that none of these are SBR machine guns, and that's true.
You can argue that you can build a 'mil spec' AR15 style rifle for more than $600 (maybe not a whole lot more), and that is true.
You can argue that what you can build for $600 or less is 'good enough' or 'you won't notice a difference' vs the 'mil spec', and that might be true, depending on the person, uses, specific rifle and parts, etc.
But you cannot build what we all know is meant by a 'mil spec' (civilian) rifle for $600.
Now, if you want to argue about what parts constitute a 'mil spec' civilian rifle, and say that it doesn't need a 1:7 twist to qualify, or it doesn't need a shot peened C158 HPT/MPI bolt to qualify, or it doesn't need a 7075 RE with machined threads and dry lube inside finish etch, that is a different topic entirely.
I don't know why you want to make this so difficult and complicated, it's really not.