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.
I think the most expensive book I ever purchased I believe is The Art of Berserk for around a $100 due to tax and customs sinc eI ordered it from Japan. Also the book is put of print which makes it expensive. I do have manga boxsets, but these are multiple books so I don't think they account. So my most expensive books are art books.