Beyond simply checking for grammatical and spelling errors, I make stylistic suggestions. In the cases where English was not the native language of the client, this has proven particularly valuable.
I've had a great deal of experience, both at the dissertation and professional level (inclusive of published articles, IMF and World Bank working papers, and dissertations).
Fee: I usually charge between £2-4 per page one and a half/double spaced, depending on the amount of editing required