That's a problematic title, considering this Gutenberg bible is neither the oldest nor the most expensive book on the planet.
The Mormon Church spent $35 million on a printer's manuscript of The Book Of Mormon in 2017. [1]
Bill Gates spent ~$30 million on Da Vinci's Codex Leicester in 1994, which would be around $54 million today.
Even talking about printed works, The Bay Psalm Book was sold for $14.2 million in 2013.
Depending on your definition of "book", the oldest known books are things like The Etruscan Gold Book, The Nag Hammadi codices, the St Cuthbert Gospel, etc.
[1] https://www.huffingtonpost.com/entry/mormon-church-drops-35-...