If you don't mind using some used components, you can save a bit.
If you have access to a milling machine, and some skill, you can have a lower from a $25 casting. Otherwise, $120 seems to be about the bottom line, all things considered. There aren't any used ones, people hang on to them.
I bought a like-new upper, complete locally for $300.
Bare upper is $50 from LAR - seconds, but I can't find any blemishes.
Bolt Carrier Group (BCG) is $100, Changing Handle (CH) is $17
Barrels are $150 and up
Lower parts kit is $55
Buttstocks run $20 - $35, forestocks $20
Magazine is $10
So, that's $542 plus any shipping or sales tax for a complete gun
Realistically, $600. For that money you can buy an Olympic Arms Plinker model - but what fun is that??