I run The Fiction Consultancy. (Find us at http://thefictionconsultancy.co.uk.) Normally we quote by the project, depending on word count and exactly what is required. For 83,000 words of proofreading, I would expect that to be a week's work and come to $455. Maybe you can get it done more cheaply but you won't get it done better. We are the experts with years of experience in the field.