I just picked up a 1988 hard cover (withoutdusk jacket) copy of Magick and was wondering, and my apologies for my ignorance, are there any differences between each edition of the book and its relevant value? This edition cost me 10GBPs (and in good condition) and I picked it up in a London antiquarian bookstore.
There is a very wide variety. Different versions contain quite different material and so are priced accordingly. The version published Guild, annotated & edited by Grant Symonds is probably the least sought after, perfect bound - I suspect that you have this version. It usually sells for around 10 pounds a copy.
My personal favourite is the above version, though published by RKP.