I think the cheapest you could go would be a little over $500 with new parts. I would check out http://jtdistributing.com/index.html and del-ton.com. There is also model1sales.com but I have heard good and bad things about them. Jt is where I have gotten my kits from, If you can get someone else to build one too and the price comes out to be $1000+ they will give you dealer pricing which gets a basic kit down to around $420 dollars.
The cheapest place I have seen for lowers is essential arms and they are $76 plus shipping and transfer fees, but you can probably find a lower at a gun show for $100 out the door. If you have any other questions just ask.
i would say the best you can do is like $500
but if you shop around and buy all the parts used from really good deals i guess you could do it for 400 but i wouldnt want to do it.
compleated lower-at a bare minimum $200 with crappy no-name parts
compleated upper-$300 at a minimum with no name crap parts or heavily used and worn parts.
i would save up and spend around $700 and that should get you a good gun.