If we are talking of dotations, you are right, as dotations are simply the fraction of the taxes that are attibuted to a certain office. they are more or less spent on the spot to pay the civil and military house.
What Digby and Susan are writing about (and I apologize to them if I understand wrong), are the "majorats de propre mouvement". It was possible for a person to constitute a majorat with his own property, situated in France, to create a protected capital that would devolve to the holder of a title or office. The last ones were abrogated in 1923.
The majorats de propre mouvement which were instituted by Napoleon himself, starting in 1809, (he had to change the code napoleon to do it) to support dignitaries in keeping up the required status, were always constituted by the income from real estate outside of France, with the express stipulation that it had to be spent on a lifestyle in France, and in Paris especially.
The actual income generated by such a majorat could differ enormously; Davout had a huge one in the Duchy of Warszaw, but never managed to draw an income from it; Ney had much smaller ones but situated in areas in Germany that did generate a steady income. We are talking of up to several million a year for each of the big dignitaries, and thousands of gold francs for the smaller ones, such as the simple soldiers who were made Comte d' Empire for bravery (Putigny at the siege of Ratisbon for instance).
There are several detailed studies on the majorats, the most recent one is -I'm writing from memory- in Waterloo, lieu de souvenir. There is an article in the dictionnaire Napoleon too.
Best regards,
Bruno