If you are willing to forego name recognition, ghost writing a book is a plan to make money. There are many "would be authors," who have a story to tell but do not know how to go about telling. Of course, there are some duds out there, too.
Word of mouth is the best way to approach ghost writing. I have worked for several who had a story to tell, and insisted on doing it their way. They wanted no advice about punctuation, etc. All they really wanted was a typist. So beware of these people and if you want be their typist, at the very minimum of $1.50 per page. That is a very cheap rate, but affordable to some.
Some others want an "editor." I find that when people discover that I am a writer, they bring out all the stops, and want me to edit their book, for free. I refuse to do that, as my time is important and taking me away from my own writing.
The true person that wants a ghost writer is one that has something to say and really wants to tell their story in the best possible light, good sentence structure, punctuation, etc. When you connect with this person, you need to look over their material first. See if it warrants writing. Based on the number of pages in the book, etc., you set a price. Remember you are not getting any credit for the writing, but you are spending your time. Time is money in the writing field, so figure your worth per hour. Figure out how many hours you feel it will take to work on this book. Some ghost writers charge by the hour, others by the page, etc.
Re-writes should be charged separately, as they are unexpected, when you ghost write. Everything has to be okayed by the writer, so be aware that there will be re-writes.
If this is your forte, you will become successful by networking with other writers to find your clients. Word of mouth is your best marketing ploy.