All his dachas were government property. He couldn't pass it on to his children. He had some sort of modest salary and savings, but not much. It might be hard to comprehend by people raised in capitalism, but Stalin goal was wellbeing and better life for all soviet citizens, not personal gain.

He made significant amount of money from royalties from publishing of his works, but that what was used for the "Stalin's awards" in art, music, literature, science etc.