$450 AR-15 - and some better options.
I was just poking around on Palmetto State Armory's site. You can get everything you need to build a very decent AR for just over $450. $400 gets you a build kit.
AIM Surplus then has lowers for about $40.
You'd still need a rear sight and mags. But otherwise, that's a lot of quality and features for not a lot of money. I'd probably rather have a Chromed barrel than stainless, but still...
Here's some other, better, options (as of October 8th):
PSA - Blemished 16" M4 - $350
PSA - 20" "M16" upper - $409
PSA - 16" Midlength CHF - $419
(all of these uppers have had the $99 Bolt Carrier Group added to make price comparison more fair)
Of these three the 16 middy would be my first pick, then the 20", and then the M4.
BCM - 16" Midlength Lightweight - $500
Configured with bolt, charging handle, magpul handguards, and a BCM compensator
This is an amazing deal. BCM has a stellar reputation, and rightfully so (please go read this link, BTW). This upper is about as close to perfect as you could want for general purpose/home defense.
LPK with the Quality Milspec Trigger - $65
This is a great deal on a very decent trigger. There are nicer triggers, but they usually start at about $100 and go up from there.
M4 Stock Kit - $50
You can find cheaper stocks out there - but not a lot cheaper. You can also spend a little more to get a kit with Magpul furniture. Is it worth it? Maybe.
All you need to build a functioning AR-15:
Upper Receiver Group (Upper)
Bolt Carrier Group (BCG)
Charging Handle (CH)
Lower Parts Kit (LPK)
Stripped Lower Receiver (a stripper lower)
Some uppers come with BCG and CH, so be aware of that - you don't need duplicates of these items (though it would make subsequent AR builds cheaper...).
INSTRUCTIONS ON BUILDING
I built my first AR with nothing more than a pair of vicegrips (for the difficult roll pins), a hammer, and I think a pair of forceps (for the detents and other tiny parts), and did so in a little over an hour.
Since then I've added a few punches and an armorers wrench. I also have an action block, but that's only needed for making relatively major changes to the upper. The instructions I used are still to be found over at Ar15.com - HERE.
ITS Tactical - This writeup is a bit on the excessive side, but still appears helpful.