The 338 Lapua is a military round, as it the .308. The Lapua is a little larger (but not much). So why does the 338 Lapua ammo cost about $2.00 per round and the .308 is half that price or less?
Usually once the military adopts a round it gets cheap. Witness the 9mm and .45 vs. 357 Sig and the 10mm.
it's all about supply chain. The military is adopting the round, but that doesn't automatically result in massive amounts of back support for the round. It'll get cheaper but it will take years, whereas the 7.62/.308 has been around for decades with plenty of civilian and military use to warrant massive production of the round.
Ohhhh the pains of adopting a newer round.