It varies a lot, but freelancers often work for a fraction of a cent per word, maybe around .2 or .3 cents. Some work by page count or other means. I paid about $3k for a very long novel.
Also depends if you're looking for a full-on editor, just a proofreader (or eventually end up with both).
There's also cover design, which can run a few hundred to a thousand.
And if you're not going strictly digital, you may need to hire layout services for the book, which can be another several hundred to a thousand.
So, roughly speaking, a short book might only cost you a few grand, a long book with lots of extras, or multiple revisions, could easily run five grand or more. And that's assuming if you print it's on-demand (the most expensive in the long run, but cheaper than shelling out for a large batch up front).