This is stupid but for the sake of being realistic. She is the top medical ninja in the village. Ninja get their own set of Income with bonuses from Missions. Being the top medical ninja would have Sakura mostly working from within the village in a hospital as it's head. In the real world, the head of a hospital makes ridiculous amounts of money. Hokage is like the president.. so Sakura would be one of the most skilled and renowned doctors whom happens to be close friends with the president. I'm sure she is well off.