You want cheap, get a Saiga.
I like Saigas and would like to get one, but they ain't an AR. A cheap AR will out-shoot a well put together AK. And magazine cost after about 10 mags makes a home-built AR-15 a bargain.
To the OP: I have a self-assembled AR-15 from a Cavalry Arms complete lower for $125 and a Del-Ton complete A2 20" chrome-lined upper for $450. Total package was $575 + shipping + magazines.
Fit is very tight, but I have yet to get out and shoot it. The 110+ degree weather has hit us here, and it just wouldn't be fun until the weather cools. This is a bummer because I also have a new red-dot for a Camp 45 to sight in and also a match-tuned custom M14 coming out of the gunsmith's shop and to me in July.
Lots of toys for this fall though.
Don't discount building it yourself. All the "assembly" I did was snap the lower together to the upper with the two take-down pins. You save the Fed Excise Tax on the entire cost of the upper, which is the lion's share of the firearm. You definitely get a lot more gun for your money. Shipping negates the sales tax, too.