For the OP:
If you want to slowly build over time, just check out the group buy section for lower receivers or 'lowers'. I think Minuteman Armament has extra Aero Precision lowers, perhaps some uppers too. Andersons are a good value also, but the Aero finish is a little nicer. I would argue that an Aero forged lower will perform as good as any lower on the market.
Once you have the lower, you have the only part with a serial number. Everything else can be ordered online. I've even seen various AR parts at a Walmart in Washington state. You don't have to 'register' until within 7 days after you complete the rifle. You can collect the rest of the parts over time, budget permitting with no hassles. The nice thing about AR's is that if you have a great barrel & matched bolt plus a quality trigger, you can skimp on everything else and still have a tack driver.
I just finished a "ghost gun" AR-308 with an Aero upper/lower combo. Yes, it's totally registered and legal, but I say "ghost" because I took 1-1/2 years to collect parts and build. Therefore, it was invisible to the family budget.
Anyway, if you are patient and look for deals, you can build something that rivals the high end stuff for much less.
As others have pointed out, for an everyday carbine the most cost effective route would probably be something already complete. But what's the fun in that? These things are like adult Legos. Here's my suggestion for chronological ordering of the major parts:
1. Lower receiver. Not necessary to match, but nice to go ahead and get combo upper lower same brand.
1.5 Next get a barrel with matched bolt whenever you can afford. Almost all of your accuracy and reliability comes from these. This is where you spend the money. I highly recommend Ballistic Advantage in Florida- they make for some of the top end AR builders. Go ahead and get the gas block and gas tube at this time.
2. LPK (lower parts kit, I like Spikes)
2. Trigger- you don't need a $200+ match trigger but at least get a RRA or Jard level of quality.
3. Upper receiver if you don't already have. Get one complete or order the parts kit now- they are not expensive
4. Bolt carrier- get the M16 version (it's OK and better...) Get remaining parts you need if you already have a bolt. If not, order as a BCG (bolt carrier group)
5. Fore end and compensator to complete the upper. If you live in Mass, don't forget to bring the fore end barrel nut and your gas block when having the comp pinned and welded.
6. Butt stock and matching extension tube, spring, and buffer. Get what you like. Sometimes these come together as a kit.
7. Sights / optic to suit your taste.