1. Charge what you think you are worth. This is an issue about your self-confidence. Some people can't bring themselves to ask for a large amount and you may need help with setting your fees.
2. Contact other collaborators and ask them what they charge. Some won't tell you, of course, but most of them are open enough to give you a price range.
3. Base your rates on your experience and background. That may be obvious, but a few first-time ghosts want to charge authors the upper-end rates. A given in the publishing industry is that you start low and as you build your resume, you increase your rates. That's the path I followed.
4. Once you decide on what you will charge, say it upfront and without apology. If you feel the amount you want to charge is fair, don't negotiate for a smaller amount.
5. Provide a contract, even if it's an informal agreement. (If you want a sample of a covenant, you may email me.)
6. When you write an article, you usually do it on speculation. You send a query letter to prospective editors, and if they say, "Send it," you have no guarantee that they'll buy the article. That's what we mean by speculation.
However, if you write books, don't write on speculation. It's too big a job and too much work. Write a book proposal and charge for writing it.
Think of it this way. The client (or author) needs to take as much risk as the writer. If someone wants you to write a book, that person needs to be able to pay you.
Writing for others involves being paid for your work.