Value of 15th Edition Encyclopedia Britannica?

I would like to know the value of an old Encyclopedia Britannica 15th edition 1987. It is in mint condition. There are 55 volumes. The books are black leather, hardback, with cushion covering. The set includes:

29 encyclopedia volumes
a 2 volume index, A-K and L-Z
1 volume index on how to use the set (Propaedia outline of knowledge)
7 Britannica World Data Book of Annuals 1987-1993
5 Britannica Book of the Year Events of, 1993 - 1997
11 Annual Science and the Future Volumes 1988 - 1998

By Lisa from UK