Basically the budget structure is defined in application documents for each Call for proposals. It's structure tends to bi simple and concentrated on just few categories.
The main problem customers have is defining budget lines which are not directly connected to actions aimed to resolving their problem or establishing new situation/service. Indicative examples are expenses for visibility of project and donor/s, travel and accommodation costs, organization of meetings and events, etc.
Hidden expenses are coming to the surface during project implementation and all of them have to be covered by lead or other project partners.