Yes, they do lose books, that is what the declared value is all about. It's exactly or that and nothing else. So say, you send in an amazing spider-man 300 and select the modern tier stating that you think its worth 200$, then if they lose it they give you 200$. So people should not undervalue their books because that is what CGC uses to pay you back. I havent heard of cases where they argue the book's worth compared to what you state. And writing a really obscene price just makes your book go through the walkthrough tier so there are far less chances of it being lost.