Keep it simple and charge whatever your league mandates for league/cup matches - if you can't get hold of anyone from the league, I'd suggest googling their website and seeing if they have a downloadable handbook on there, which should set out fees. Do whatever you can to avoid guessing, as that will only cause you issues if you turn up later in the season having worked out what you should have charged at that point.
For OA that should give you a straightforward answer - for youth it can be a bit complex as most leagues will have a sliding scale depending on what age group you're refereeing in.