I assembled my AR in that exact fashion. Only problem is when I completed, I tabulated total cost (saved all bills and reciepts) and it cost over 1000 bucks for something about 800 bucks in concept. However, I could afford it over a year, rather than all at once.
I personally feel the gun industry caters to the 'impulse buy". You have to buy it complete idea is complete gibberish. Build the rifle YOU want not what a COMPANY wants you to have.
In all seriousness, if one has patience (a HUGE key to a project like this) then they can have an awesome AR. I couldnt afford 800 bucks down pat, however, I could afford 50 a week. Save up a couple weeks: barrel. A couple more: Reciver (upper or lower). Etc etc... Patience is key to this. In todays, get it before its banned world, we have to sometimes take a step back and remember that we are rewarded with our perserverance. Instead of copping out on your AR project and getting another .223 rifle, like a Keltec, gather patience and keep building the AR, the gun you ORIGINALLY wanted.