The book you have is one of the two books that came in the Companion boxed set. The other book is the Players Companion. The box and all three books have the same basic graphic on the cover, just the wording is changed.
http://www.acaeum.com/ddindexes/setpage ... anion.html
The companion set is rather common, and depending on condition might go for $5-15. By itself the DM's book would probably go for $5 or less, unless you get lucky.