Length of the Book
Obviously, the length of the book, (word count), is the biggest factor. A 20,000 word book will cost roughly half of a 40,000 word book. There is some reduction once a book gets into the 75,000+ word count range, but those sized books are few and far between in the ghostwriting world.
Research Involved
The research involved in writing the book, whether it is fact-checking, fact-creating, or historical confirmations, takes time to get it done right, and thus, has a cost to it.
Special Requirements
Special requirements will play into the overall cost. These include changing direction of the book once more than about 3 or 4 chapters have been written. As well, creating a special language used by the characters.
New York Times Bestseller
New York Times Bestseller. If the author is already a known entity, a celebrity, or otherwise someone whose book would likely become a NYT Bestseller, we are certainly willing to take that into consideration and charge accordingly. Even though my name will never be ‘known’, having a few current NYT Bestsellers under my belt every year is a strong selling point for my work, and thus warrants some fee reduction to those authors who can add to my (Invisible) notoriety.