Building an AR from scratch is both easy and rewarding. It is so easy that my 12 year old son did most of mine with me setting there. I am losing my eye sight very rapidly or my arms are shrinking and I can't get anything out there far enough to see anymore.
I picked up a stripped lower for $60 from a co worker who has an FFL, I got the lower assy. kit with a extra complete spring kit thrown in at a gun show for around $30 (I think) and I got a VHS tape with instructions for $20. This tape is awesome, it goes into great detail. You can get a parts brake down off ar15.com. I have since bought several uppers and swap other uppers with my other AR's. AR's Rock.
Oh, if memory serves me correctly I built mine for a little less than $400 but I bought a used upper for pennies on the dollar.