I work according to an hourly rate (350 Kc an hour) as this is generally 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). 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 six page text which needed substantial revision. As a general guide, a scientific article of approximately twelve double-spaced pages usually takes between one and two hours to correct (therefore, between 350 and 700 Kc). A twelve-page article from the social sciences or humanities usually takes a little longer – typically between 1.5 and 3 hours depending on the level of English (therefore, between 525 and 1050 Kc). Of course, longer texts will cost proportionately more, and shorter texts proportionately less (although, for practical reasons, I charge a minimum fee of 200 Kc).
I always correct to the highest level, grammatically and stylistically. Such correction might range from the addition of missing articles etc. to the complete revision of whole sentences and paragraphs in order to rescue meaning.