Where can I find a cheap (but good) ghost writer?

If you have a highly commercial book idea, or are a recognised authority on something important, or have a large online platform or following, then your up-front budget is largely irrelevant because you can offer a percentage royalty on the sale of the book. If you don’t have any of those things then all you can realistically get for $400 is a 20-50 page book. If that’s the size of book you’re after, just look around on Amazon for authors who have written books similar to the one you’re thinking about, and approach them, offering them $400 advance and 10% royalty. You only need to chance on one of them who’s a bit skint, and they might take you up on the offer.

Another way to go is to first get a recognised book publisher to accept your book proposal on its own merits, based on the size of your platform, your expert credentials or the sheer marketability of the book synopsis. State in the proposal document that you will be using an experienced co-author.