I'm guessing the reason for the expense is they are low print runs. The Bible industry is massive, though like most high end items, this is more of a niche within it. About 20 million bibles are sold a year in just the US. The bible translation industry is also massive, which feeds into it because you want the most accurate (readable, beautiful, whatever) translation you can get, approval lists from churches change to reflect that. There are currently around 50 regularly used English translations available, and books for all of them, but some are obviously more popular like King James or the New American Standard.
TLDR: It's yuge.