I've just completed a build last month. Here's a pic:
I built this on an "M15" Armalite/Eagle lower I purchased through my FFL from Quantico Arms for $75. The entire upper, buttstock, and pistol grip are all A1. The upper is one of 16000 originally ordered by the Air Force and doesn't have a forward assist. I pursued this very particular model on purpose and wanted those components. So, in this case, it was justified to build. It was amazingly easy, btw.
If I had decided just to buy one for the fun of it, however, I could have saved a few hundred dollars by picking up a "varment" rifle. There are many advertised in the $450-$500 range all day long. And, yes, you get a warrantee so if you want to shoot quickly and have something that's been assembled, buy a complete rifle.
If you want to assemble one yourself, however, you can do it realtively cheaply unless you're wanting one like I've completed. I can highly recommend eBay as one source for parts. Just type "AR15" in the search slot and you'll find all kinds of uppers, parts, assemblies, slings, etc., etc. The great thing about this rifle is that virtually all the parts are interchangable. It is amazing. Just understand that older, military uppers sometimes have the large front pivot hole and all the retail, civilian lowers have small front pivot holes. This can be remedied but sticking with AR15 upper will eliminate that issue.
To build, just make a complete list of the parts you'll need"
1) Complete, barrelled upper
2) AR15 Lower from someplace
3) Lower Parts Kit (h,t,s, springs, pistol grip, pins, detents)
4) Buttstock assembly (depending on your model)
5) Bolt Assembly (complete)
6) Charging Handle (complete)
That's it! Now just price them all out from the various sources and once you've got all the parts, it will take the average guy under an hour to put it all together. There are no special tools needed if you get the upper assembled which I would highly recommend.
Finally AR15.com is a fine source for information. Virtually everything has been discussed there at one time or another so just do a search or spend a couple of hours browsing through all the forums there and you should find all the information you need to find about building.
Hope that helps. BTW, my new rifle shot extremely well out of the box.
Rome