I'd say it's complicated because it's so subjective. It's up to the individual editor and publisher to decide how much a book is worth. And the variations in the views from one editor or firm to the next can seem to make no sense.

Often it comes down to something like an editor's "instincts" about whether or not a book will do well and whether he or she can sway whoever else may be involved in the decision-making process. How do you put a dollar value on "instincts"? A lot of publishing houses try to do it by looking at "comps" (comparable books), but even that isn't foolproof. Each book is unique.