Finally, when pricing these things you need to make sure you're comparing apples:apples. You need to start with a
BCM BFH 16" w/ DD Lite 12.0 at $949 (note that BCM does sell their uppers with the KAC URX but it's
$300 more. From there you need to add a
BCM BCG for $150, a charging handle for $20+/-, and a set of MBUS (since we're going on the cheap) for $100. That gets you to $1220 for a complete upper.
Moving to the lower, you're going to start with a
BCM at $330 without stock, add an Emod for $120 (going for that "same function, reduced cost, the SOPMOD is another $80), and a
Geissele AR15 Super Semi-Automatic Trigger for $170. That brings you to $620 for the complete lower and up to $1840 for the complete gun. Bearing in mind that you still aren't getting the E3 features.
I found a KAC SR15 listed on Gunsamerica for $2200, which is a $350 premium over the package above. What you're getting for that premium is the E3 bolt features, the ambi lower controls, and what is probably a lighter package overall. Put a set of Troy BUIS and the SOPMOD stock on the BCM and you narrow that gap even more.