That $470 isn't for an upper, its an entire rifle, minus the stripped lower.The Annoyed Man wrote: $80 includes the lower parts kit? DPMS parts kits generally run about $69.95. What about sights? 3 or 4 magazines, even cheap ones, are going to set you back another $50+.
I was thinking too that lowers are going to be scarce right now. Maybe I'm wrong.
http://www.del-ton.com/Rifle_Kit_p/rkt101.htm
Now, there are several things in that kit that most people don't find desirable, primarily the non chrome lined Heavy barrel and 1:9 twist rate. But minus a rear sight, that $470 plus a stripped lower will get you an entire rifle. I haven't checked stripped lower prices in a while, got mine for $50 last year during a Palmetto State labor day sale. Generally I've seen them for ~$80 though. So add another $40 or so for a detachable carry handle, $20 for FFL fee for the lower and you're looking at right over $600 for a complete AR. Throw in the magazines as you mentioned plus enough ammo to fill them and you're looking at closer to $700.
But the point was: Yes, you can build a ~$600ish AR.