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.