The short answer is: it depends.
I KNOW. I know, that’s really annoying. The long answer is that my baseline for very basic proofreading is US$0.03 per word. This gives you a ballpark figure to go on, and I apologise that I can’t be more specific.
The reason I can’t give you an exact amount is that I edit books in all stages of completion. Sometimes it’s the first draft, in which case the writer usually wants me to focus mostly on content. Sometimes it’s the final version before the author publishes, in which case I’ll be mostly proofreading. Sometimes it will need both copy editing and content/structural editing. It really varies, and it’s something I need to discuss with you, the writer, on a case-by-case basis.
I know that for most writers, budgets are small and you’re constantly keeping an eye on costs, but if you’re really serious about wanting your work edited, then please contact me at postscriptediting (at) gmail (dot) com and we can chat about what you need, and what I can offer. After all, emailing me costs you nothing! And more often than not, we can work out something which will suit us both. So please, get in touch!