I just built my first AR. I wanted cheap since I just finished my vepr build.
Used olympic SUM .223 upper with BCG $350
NFA Poly lower (complete) $100 OTD locally
$450 for a complete ar.
Shoots good too, I am getting 1" groups easy at 100yds with a cheap leapers scope.
I would find a cheap upper online since you can ship it, save tax too.
The NFA poly lower came complete with everything. Its nothing fancy but it works. Me and a friend shot about 200 hard rds some russian steel case and that lower didn't flinch.