Are you looking for a general editor, a mechanics editor, or both?
In my personal experience (working as a freelance edtior and paying them) 1/4-1/2 cent a word seems to be about standard for PDF stuff. Print seems to command a higher price.
As for mechanics editing, I tend to get a bit more for that when I do freelance work. Somewhere in the 1-3 cent range, depending on the complexity of the assignment.
Of course, these are all just general guidelines, and the rate depends a lot on the individual skill and experience of the editor.
Hope this helps.
BD