Why do you want a cheap AR?
The cheapest I've ever seen an AR was ~ $650, and that was a Del-Ton kit build on a stripped Aero Precision lower with a MagPul MBUS rear sight. Shipping, FFL transfer fees included. Nothing fancy to it at all, but it worked okay.
For a complete price estimate of every single part, and where to get it, to put it into a solid, professional-quality rifle:
$400 -
BCM upper receiver assembly (no fancy rails)
$90 - Stripped lower of your choice of manufacturer (most are about the same quality anyway)
$94 -
G&R Tactical lower parts kit with MagPul trigger guard and stock trigger
$65 -
Daniel Defense A1.5 Rear Sight
$65 -
BCM M4 stock kit w/ mil-spec receiver extension
$130 -
LMT Bolt Carrier group, auto
$5 - A2 grip (at least that's what they go for around here)
$45 -
BCM Gunfighter charging handle
$34 -
MagPul MOE midlength handguards
Free - built using
the instructions found here.
That's $930 for a rifle that is a heckuva good compromise between carbine maneuverability and rifle handling and accuracy, and can handle range use, competition, carbine classes, and war that will last thousands upon thousands of rounds, and is compatible with every quality spare part made for the AR platform.
I've built both the Del-Tons mentioned above and (very nearly--the only real difference is the UBR stock) the mostly-BCM rifle listed. The Del-Tons were fun, but the BCM ran through a local carbine course of about 1100 rounds (tallied 37 magazine changes
) in two days with
no cleaning, generous oil, and Federal XM193. Had zero problems even with Brown Bear 55gr, whereas my Del-Tons didn't run as reliably or as accurately.
It's quite possible to get solid, dependable quality for not a whole lot more, with a little searching... and I've kinda already done that for you.
Besides, the real cost of owning an AR is in the optics, ammo, magazines, and so forth, but mainly those three.