Google it. "Fidel Castro wealth" search turns up with various claims ranging from a net worth of 100 million to "more than 900 million $". And that's just a small part of what he had access to.
When you are the sole and absolute ruler of a country, you have access to all the wealth of the state and its people (which are fucking poor) and have little to no accountability.
.Do you really think he was as poor as all the other people of Cuba?