Excluding textbooks for classes, I bought this for ~$300 (several several years ago as a special treat)
Most of my collection is thrifted/bought from garage sales (for a quarter to a dollar usually) but I do have a handful of nicer ones.
Depends on definition of book. I have bought (and read!) the Encyclopedia Britannica. Was 7000DM, about 3500 dollar plus 40 years of inflation.