Some people charge by the hour. I started that way, but I soon learned that authors want to know the total cost for a project. If you must work by the hour, set a ceiling and say, "The total amount will not exceed . . ." It's easier (and I believe more professional) to set a price for the total work (excluding travel).
Today the fees to write a book run anywhere from $2,000 to about $25,000 for experienced ghostwriters. One mid-sized publisher told me they start flat-fee ghostwriters at $7,500. The top-paid ghostwriters earn six figures—and there aren't many in that category.
If you charge a fee, how do you estimate it so that you won't lose money? That's where you consider your experience, and you need to analyze your work habits. Some writers are fast (I'm one of them), and others are slower—neither is superior, only different.
Here's how I approached flat-fee arrangements. I estimated that it takes four months (full time) to write a book. I would say to prospects, "Here's how much money I need to make in one year," and I give them an amount. "I'll sell you one-third of my professional time." Although they always understood, not everyone was willing to pay my fees.
One final thought on setting a one-time fee: If it takes longer than you estimated, you lose. If you do it faster, you gain. I've experienced both—and that was part of my learning.
My responsibility is to decide how much to charge.
That may not be easy to figure out,
but it's part of being a professional.