USD 295 from Amazon
This book is considered to be the authoritative reference on the Latin language, giving citations for every bizarre usage of every single word, and context to go along.
As a first-year classics major, I was unable to justify this one to my parents; indeed, my instructor has said that even at the graduate level, a much cheaper and less comprehensive dictionary such as Cassells, combined with access to the one in the university's classics library is sufficient.
But I want it. I want the strange, never-used-except-for-maybe-once-when-Livy-was-drunk definitions created in the great Roman tradition of stretching the meaning of a word beyond all recognition, rather than allowing a new one to be created.