Seeking a general idea as to cost to edit a paperback book?

For those who have real skill in structure, grammar and punctuation of the English lanuage. How much would you, if you did, charge to read, edit and fix such issues? Secondly I am in the market, sorta speaking, for such a skilled person. The book is basic normal paperback size and contains a little more than 300 pages. I know that there are many such mistakes in my book. Mainly am just wanting to get a basic idea of what I'm looking at here. As well as an after thought am looking for someone skilled in this area.