what i did when i bought/built my AR: bought a Rock River Arms assembled lower with collapsable buttstock and their two-stage "national match" trigger group at the local show in Houston (high-caliber gun show) for around 380 mainly to get the background check over with, but the complete lowers were actually reasonably priced, complete uppers were all marked at retail+.
then i ordered a DPMS 5.56 NATO carbine complete upper from midwayusa.com for around 400, pinned it to the lower and mounted an A2 style BUIS, and went to the range to break it in.
far as i can tell, if you want a budget build, its actually cheaper to buy a complete upper/lower online than to build a complete rifle from parts, unless those parts come in a complete kit. you might be able to get an upper receiver, bolt carrier group, and barrel for less than a complete upper, but then once you buy gas block, gas tube, forearm/handguards, charging handle and flash hider, you're going to be 100 bucks+ over what you could have bought a similar complete upper for.
i've recently starting thinking about a 6.8 remington spc upper for my RRA lower, and for the 500 bucks or so that they start at, i'd still be short quite a few odds and ends that are needed to complete the build. it's only when you are building a high end gun that you can really save money building a gun piece by piece.
