If we are talking about expenses, it’s different from one area or country to another. You have to look online at the price of hotels, hostels, restaurants, and look for blogs, etc., for an area. It’s obviously different in Senegal than it is in London. Fixers are $75 a day in some places, and $200+ in others. And don’t forget the roles of driver and translator, which may be included in the fixer role, but not always.
A good rule of thumb is to create a budget based on the best info you can get, then double it. Then bring twice as much money as the budget (only a slight exaggeration).
If you are creating a budget for a client assignment, always err on the high end for expenses.
In short, it’s not magic, and you just have to do your homework diligently…there are very few rules of thumb that you can apply to get anything accurate. It can take days to gather the info you need to plan even half-way rationally.