You can put a stripped lower and parts together for cheaper than buying a complete AR plus you can built it exactly how you want it without having to buy a complete AR, take off the parts you don't want then buying the parts you do want seperate.
The lower receiver on an AR is what the ATF considers the "firearm;" all other components are simply parts and not subject to form 4473 and NICS checks.
The buying frenzy on AR lowers is a result of paranoia over the election and fears that they may soon be banned with a new AWB.
If "assault weapons" are banned, stockpiling lowers is far cheaper than stockpiling complete weapons; the assumption being that other gun parts will not be banned. Trouble with that thinking is that unless you have all the parts in your possession prior to any ban that funtions like the previous AWB and can claim you already had an "assault weapon" grandfathered in, you will be creating an "assault weapon" illegally.
I had thought about getting a couple back in June but didn't . One place I looked had some for $78.00 and if I remember correctly, DPMS stripped lowers were around $100.00 but as Kbear said, not anymore.