Let's say they paid X for the book. They had extra work with it, to prepare it for library use, so that's an upcharge. It was on offer in the library for people to use for X years, so that's a downcharge. Depending on the state it was in, that might also influence its value. And then they have their own calculations (which I don't know) that calculate the fine. Then based on the popularity of the book, the number of copies they have, its importance to the library etc, they may or may not get a replacement.

So you're paying for the book you lost (and how much it was worth to them), not for its replacement. (So for this book you might pay a 200€ fine, not 1500€ for the ebay replacement)

But again, this is VERY library dependant.