1. Find a cheap upper. (not too cheap.)
2. Find a cheap lower. (not too cheap.)
3. Pin together.
I got a New Frontier Gen II upper from Joe Bob's for $95 shipped after a coupon and a Del-Ton DT1026 dissipator heavy barrel for $384.54 shipped from Midway after a coupon.
Yes, you have to shop around. :)
My point is this: have your cash ready, and shop, and you can put together a decent AR for under $500.
I also got a deal on a CQB sling for $9.95 shipped.
I compromised on weight by getting the heavy barrel for more stability/accuracy. With an unloaded 30 round metal magazine, it weighs 7 lbs.
It is nose heavy with the very light weight lower, but a heavier and loaded mag will help that.