They are expensive, barrel contributes to that...
I think the cost of pre-assembled uppers is also higher due to lower volume production. They're not cheap to shoot, and most AR shooters aren't into the long range game, which is the whole reason for the existence of the Grendel.
My son wanted one, we decided to just build it- from scratch, stripped upper and lower.
He had no previous experience- even with most hand tools- yet handled it with aplomb, by himself, in one long evening with the help of Zedicker's book and Brownell's online video series.
Total cost for the rifle- upper and lower (excluding vise blocks, etc), was about $1100.00 with an 18" ss barrel, including an $80 flash hider, and YHM free float handguard...
You could build one yourself for $600-$700. Biggest expense is the barrels.