It seems to be quite a scarce book, the only one he wrote and there are collectors of books based on TV series especially if they are signed.

This has probably led to some optimistic pricing on behalf of sellers, its impossible to say whether the price is justified and copies do sell at the high prices or whether they have been sitting there unsold for a very long time.

A book can be scarce without being valuable. A book is only valuable if someone wants to buy it.

That doesn't really help you much I'm afraid.

If you did list it I would go for TV Memorabilia as a category rather than fiction books and try an auction to see what happens.