Hmmmm..looks like you could buy a mid length upper with a spikes 9" tactical rail and build he lower with a CTR stock, MOE grip, AGL combat trigger group and Magpul BUIS for less than the one in the add....unless...ya didn't feel like it and just wanted to buy and go shoot...which is what most people want to do.
Thing is though, once you build one or two, unless your buying some HIGH END, like a HK or LaRUE or LWRC or something, you get a little spoiled with being able to build it the way you want, but buying the upper complete...just too easy nowadays :-)
This is my first ar build, the lower is finished. The spikes upper looks pretty good for the money, I guess that's why they are hard to find. May just spend the extra and get the BCM. Decisions, decisions. I never thought I would ever want an AR but thanks to Mr. O'malley I decided to build one and I can see how they can become very addicting!!