How does a book's condition affect its price?

Before I start, apologies if there's already a similar thread in here but I couldn't find it, in the "New Boards".

I'm trying to find out if there's a rule of thumb that booksellers follow to price used books on the basis of their conditions.

What I found on the web is that, generally, books are classified this way (please do correct me if this is wrong).

• New: In perfect conditions, no signs or marks, never opened.
• As new means that the book is in the state that it should have been in when it left the publisher. This is the equivalent of mint condition in numismatics.
• Fine (F or FN) is "as new" but allowing for the normal effects of time on an unused book that has been protected. A fine book shows no damage.
• Very good (VG) describes a book that is worn but untorn. For many collectors this is the minimum acceptable condition for all but the rarest items. Any defects must be noted.
• Good (G) describes the condition of an average used worn book that is complete. Any defects must be noted.
• Fair shows wear and tear but all the text pages and illustrations or maps are present. It may lack endpapers, half-title, and even the title page. All defects must be noted.
• Poor describes a book that has the complete text but is so damaged that it is only of interest to a buyer who seeks a reading copy. If the damage renders the text illegible then the book is not even poor.
• Ex-library copies must always be designated as such no matter what the condition of the book.
• Book club copies must always be designated as such no matter what the condition of the book.
• Binding copy describes a book in which the pages or leaves are perfect, but the binding is very bad, loose, off, or non-existent.

Now, how much depreciation would there be between one category and the next, as a rule of thumb?

Do used books immediately drop to 50% of their original price when new by virtue of being second-hand? Or is there a "degradation" scale?

I'll admit I've a vested interest in this, I bought a hardcover book on-line, a collector's edition, which was supposed to be brand new. However, I got a second hand copy with some signs of damage on its spine in two places, and it had obviously been opened, because opening the cover wasn't as "crisp" a feeling as it would have been when new. Its conditions would be, at best, very good. I was offered a 30% discount - is that a cop out, or is that a reasonable offer?

Thanks.