I've been crunching numbers for months. Even did last night at the Fire station. The absolute most cost effective cartridge is the 7-300, loaded with H-Retumbo, and a 195 gr Berger EOL. Trust me, there's not a more cost effective way to do it.
28 Nosler brass is very high.
.338 LM, everything about it is expensive, and it ain't all that at a mile, plus.
.50 BMG, and cases from it, aren't doing anything more than delivering more energy, and again at a high cost of components. Hell for .50 BMG you need a special press and very expensive dies. .300 Norma Mag loaded with a 220 gr ELD-X runs below 1340 fps (the real velocity to look for, for stability) 200 yards before the 7-300 And the 195 EOL.
Go dig for yourself if you think I'm wrong.