The short answer
Most professional memoir and nonfiction ghostwriting falls between $5,000 and $60,000+, depending on the writer's experience, the book's length, and how much collaboration is involved. That's a wide range, and the reasons why matter more than the number itself.
What actually drives the price
Three things move the number more than anything else: the writer's track record, the depth of collaboration (a few interviews versus dozens of live working sessions), and the length and complexity of the finished book.
Why some quotes are so much higher than others
A ghostwriter charging $3,000 and one charging $40,000 are usually not offering the same service. Cheaper options often mean fewer interviews, less revision, and a writer working largely on their own from a handful of calls. Higher-end services usually include structured collaboration throughout, not just at the start.
A better way to evaluate the cost
Instead of comparing sticker prices, ask what the process actually looks like: how many working sessions, how involved you'll be, what happens if it's not a fit partway through. That's a much better predictor of whether the price is fair than the number alone.