RTL news did some research about the costs of the Royal House. They say that there is some sort of 'smoke screen', various departments try to 'hide' costs etc. For example the ministery of Traffic & Waterstate: they only listed the costs of the royal airplane on their balance while the other travels of the members of the royal house with other airplanes are not listed. In 2001 they said the costs were 275.000 euros while the real costs were 1.1 million euros for example.
Article of RTL news in Dutch here