I work according to an hourly rate (600 Kc an hour) as this is much fairer to the client than a price per page, especially if the level of English is already good. Therefore, how much a job costs depends only on how long it takes to complete (i.e., on the length of the text in combination with how much needs to be corrected/rewritten). Thus, a twenty-page text written in generally very good English might take the same amount of time to correct (and, hence, cost the same) as a five-page text which needed substantial revision. As a general guide, a scientific article of approximately fourteen double-spaced pages and written in generally understandable/satisfactory English usually takes around three-to-four hours to correct (though this may be plus/minus an hour, depending on how many problems actually arise).
I always correct to the highest level, grammatically and stylistically. Such correction might range from the simple addition of missing articles etc. to the complete reformulation of sentences and whole paragraphs in order to rescue meaning.