There is no one-size-fits-all answer to this question. The amount you charge will depend on numerous factors, such as your level of experience, the type of editing you're doing, the length of the document, the deadline, and the client's budget.
One way to come up with a rate is to calculate how much you would like to earn per hour, then divide that by the average number of words you can edit in an hour. For example, if you would like to earn $50 per hour, and you can edit about 1,000 words per hour, then you would charge $0.05 per word.
Of course, you can always adjust your rate up or down based on the particular project or client. For instance, you may choose to charge a lower rate for a student or first-time author, or a higher rate for a corporate client with a large budget.
Ultimately, the best way to determine how much to charge is to simply ask your clients what their budget is, then give them a range of rates to choose from.
