It depends on the state the books are in. One US bookseller has Vols I-X (the complete set?) for sale for US$100:
http://dogbert.abebooks.com/servlet/BookDetailsPL?
bi=24233651
. The books are described as "Very good [quality], cloth, gilt tops, some shelf wear". Also at
http://www.abebooks.com
are individual volumes ranging in price from US$25 to US$ 5.95 depending on the state of the books.
Looks like a wonderful set of books - they could help you answer a lot of questions on this site!