I charge approximately USD: $0.005 (½c) per word for proofreading, dependent on the quality of your manuscript. This is the very last part of the editing process before you hit 'publish'.
A proofreader will detect any grammatical errors that may have slipped through the revisions after the content edit, including spelling and punctuation. You can also use a proofread to prepare for your content edit, which should result in a quicker turnaround time and a lower rate from your content editor.
I always include proofreading with my line edit and my copy edit. It could be worth your while to invest in a combo like this, instead of limiting yourself to proofreading only, depending on your situation.
Just remember your final, pre-publishing proofread is better done by someone who's not familiar with your manuscript. This will mean they're more likely to pick up on any remaining errors. Don't skip this stage because those last little typos can make the difference between a professional finish and a mediocre one.
For even more information visit my website: https://joysephton.wixsite.com/emagine