Based on a 10,000-word document.
3,250 words per hour (roughly, $0.01 per word)
Proofreading is one of the final steps in the journey to publishing a manuscript. Ideally, the manuscript should be in its final, final draft and has been edited multiple times - hopefully with a professional somewhere in there. A proofreader's job is to look for remaining mistakes and inconsistencies. Often, this comes in the form of omitted words, catching names that were changed but missed in certain spots, and, of course, spelling and grammar mistakes.
I request a 2000-word sample from the manuscript. This allows me to estimate the time it would take to finish the project and create a quote.