The usual advice you will hear is go to Palmetto State Armory for some good deals.
I got my lower from Primary Arms and I ordered a Barreled upper from CBC Industries for $250 on sale. 1/7 Twist with KeyMod floating rail.
I'm excited to get my hands on it.
So if it helps, stripped lower: $50 (Before Transfer fees and such. $80 after.)
CBC barreled upper: $250 on sale
Lower kit (Trigger and stuff) $70 at a local store
Stock and buffer tube: $110 (I went with Magpul for the stock)
engraved dust cover: $30
I haven't chosen a charging handle or bolt carrier group yet but after everything is said and done I should blow maybe $200 between the two because I want a needlessly fancy charging handle and a chrome or nickel plated bolt.
So total may be around $600-700 total for a customized AR to my specs and preference. That's VS something cheap like the M&P Sport II, Bushmaster, or even the DPMS which you will customize all of those anyway because they are all rather basic ARs and could use a few changes to make them really nice. Those are all $600-700 ARs before you start customizing them.
EDIT POST!
Christmas was good to me! (Well I mean I payed for it all, but still)
I got me a couple of toys to play with:
You can see all the info for them here:
http://imgur.com/gallery/ACPag
But for those who don't want to click:
I am almost done building my AR. All I need is the Bolt and the Charging handle. I may cheap out on those until I decide on which expensive ones to go with. I know the Archangel Charging handle is a must for the final build.
I also got the Canik TP9 SA in FDE.
I hope I can hit up the range this weekend for some fun with the Canik.
@Fawkes:
You got a really nice Piece. Colt I feel is really just riding on their fame for a large part but as a firearms expert for Aafes, I can attest that Colt is indeed one of the top manufacturers on the AR industry. As for accessories, because you got the magpul furniture you will be slightly limited on that, but as an AR in general, you can literally swap out any part of that rifle just fine. If there is anything on that rifle you feel could be better, then it can be better and you should make it better.
Also, the AUG: roughly $2000 for one of those babies. The HK91 is a little price for being an HK and you can get away with something like the PTR or C308 from Century Arms. I've shot the PTR and it's a trip. I hope you have as much fun with your AR as I know I will have with mine.