I find the simplest pricing strategy is to charge by the word count. This way, there are no surprises! I charge 1c. US per word for proofreading, and 3c. US per word for copy editing. If you refer to the following links, you can see exactly what is covered by these two services: Copy Editing and Proofreading.
Once this has been agreed, I take one third of the total as an upfront payment, usually via Paypal, upon receipt of which I immediately start work. Once I consider the work basically finished, I take the second third, at which point I send the document back to the client for their approval and so that they can inform me of any further changes which they would like to see. Once these are attended to, I will ask for the final third of the payment and, immediately upon receipt, the completed document will be returned to the client. If required, for a small additional fee, the manuscript can be turned into an eBook, suitable for submission to Amazon, Smashwords, Lulu print-on-demand and so on.