In the first part of the series, Kusaka was working class I believe, but in the last part he was wealthy. How did he become rich? How many fans were shocked when he forced himself on a depressed Akizuki in the guest house?
He became a politician in the new government, which is universally well paid. Many ex Samurais took high ranking positions in the new government because they had good educations. Remember the four years he spend studying in England?