The arfcom Build it yourself forum keeps a thread going on the latest sale and closeout prices. I found the LAR Grizzly upper blem sale there - its from a milspec supplier, mine came with a Cardinal Forge mark. $48.
Other AR forums have smaller vendors who put things up for sale at good prices, LPK's, used M4 carry handles, etc. And one thread is a swap meet no charge, put up your extras, and simply ask for what someone else is giving away. Don't abuse that one.
Brownell's has a significant Mil/LEO discount that makes them very competitive.
I built a 6.8SPC, if you are considering an alternate caliber those specific vendors advertise in those forums, and they are also great suppliers for other parts as a package. I got a 16" barrel from ARP, it came nitrided, and also ordered the complete bolt carrier to match. A bit of a splurge, but altogether a competitive price shipped in one package.
Shipping IS a significant factor, bundle orders to reduce it. I have about $50 in shipping costs, which would pay for some mags easily. Those had to come extra above the budget.
An extreme tactical rifle build with all of the state of the art parts can run $1,800+, a budget build under $500, what you get as a result should be something no one makes. Otherwise, it's cheaper to buy it complete. A bunch of parts at retail usually cost more than a rifle shipped complete.