I charge by the hour, which means the price will depend on how good the language is already, but we have to start somewhere, so my estimates for work are based on page count, and represents the maximum cost it could be. If it takes less time, however, which is often the case, then the end cost will be less.
Proofreading: 20-25€/page (40€/hour)
Editing: 25-30€/page (50€/hour)
Translation: 70€/page (70€/hour)
For audio work (music, sound recording, mixing, voice acting), it is more easily arranged on a case-by-case basis, as these can also be combined.