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.
Not that much. Probably around $30-40USD at an anime con for a manga, not including the secondary one I got that kinda doubled the price.
Otherwise, for books plural, not me, but I am sure the Christmas gift boxset of a manga series I got probably around $100+USD would be the most expensive books I've seen purchased. College textbooks excluded.