I have been researching and planning my first AR-15 rifle for almost a year now (I got time while saving up the money for it).
Originally, I was all set to build the whole thing myself, for the learning experience, and the bragging rights, and I thought it would be less expensive.
BUT, I've found that, for the setup I want, it's cheaper to buy a complete lower and complete upper and pin them together, than it is to buy separate parts and build it "from scratch" or buy the whole thing NIB. I could do a TON more research to find the cheapest parts around, but I'm not that patient: I want my AR sooner rather than later.
I've been compiling a "book" on my thoughts and experiences on researching and deciding on what exactly I want. It has helped me organize my wants/needs and thoughts tremendously. Also, check out arfcom at www.AR15.com
under the "build it yourself" section. Look to see what others have done. Also, check out www.rockriverarms.com
for their setups and prices.
My research has shown me that, for MY setup that I want:
LOWER: Receiver (Forged), Trigger Group (NM), Grip (Standard), Butt stock (Stnd A2)
UPPER: Forged A4 flattop Upper Receiver, Bolt Carrier Group, 20 Barrel (ChMo 1:9), Hand Guard (A2), Muzzle Device (A2)
$769 to buy the complete rifle ready to go
$760 to buy all the parts and build it myself
$714 to buy a complete lower and a complete upper and pin them together
Not a significant savings, but it showed me that building vs. buying isn't really a cost savings at all.
Building it yourself is all about the learning experience, pride in building it yourself, bragging rights, getting exactly the setup you want, and a better understanding about the intricasies of your AR-15.
If you just want to get it ASAP and go shooting, then buy a more "standard" setup outright.
Good luck on your decision.