You are a pro. You’ve been paid to write magazine or newspaper articles. Your books have been published books. You have skills and credentials. The a family member, friend, casual acquantance, or some guy you met at the bus stop says, “I’ve got this great idea for a novel (or non-fiction book). You should write and and we’ll split the royalties!”
As flattering as it might be to have people want you to write their stories, there are a number of things to consider before taking the plunge. The big question you need to ask yourself before embarking on this kind of project is; is this a work for hire (i.e. they are paying you for your services as a writer, and you will not share in the royalties), or is this a co-author project (i.e. you negotiate the royalty split, which may include things like an upfront payment). Whichever way you chose to go, it is important, I would go so far as to say imperative, that both parties understand and agree to all terms BEFORE you write a single word.
The truth is, everyone has a story to tell; but most people don’t have the necessary skills to craft that story into a compelling narrative that anyone outside of their immediate circle would have any interest in reading, much less buying. Some questions you definitely want answered include:
1) is this a compelling story?
2) if it is a compelling story, what kind of platform does the the client have to get the story out? (i.e. how will people find out about it so they will buy the book, or is he/she relying on you to market the book as well as write it?)
3) how is this story different than everyone else’s story? It is a war story? How is it different from the stories of the other million soldiers deployed during Desert Storm? Her child said something really, really clever and cute? Great! How is that different than what every other mother thinks about what their little darling said? What makes this story worthy of publishing? Or is it really just a family memoir that would be really cool for the grandkids to read?
The harsh reality is, there are a great many stories that need to be written, but that do not need to be published, or even read by a broader audience.
If you decide to press forward as a ghostwriter, my advice is; determine what your time and talent is worth and always get paid a minimum of 50% up front. Note: the client is the one paying you… not the publisher.
How much should you charge? That’s a subjective decision, but it is common for ghostwriters to charge by the project, by the finished page, by the word, or by the hour. Rates for a pro (which you are) can range from $30 per hour to $300 per hour and up. Or from $0.10 to $3 per word. So a 30,000 word manuscript could be worth from $3,000 to $90,000. Than might sound like a lot, but consider this: if you spend 80 hours working on the manscript (and I suspect you will spend far more than that), at the low end you are only charging $37.50 per hour.
On the flip side, maybe you see this as a ministry, and you just want to help the guy out. You can agree on a royalty split, in which case, nobody gets paid until the book is published, copies are sold, and royalties are generated. Remember there is a 79% chance that the book will sell fewer than 100 copies. The final question you should ask yourself before signing on the dotting line is; is it worth it? If it’s your book, maybe it is. If it’s not your book, maybe not.
This link will give you some additional insight into ghostwriting/co-writing.