Take more money than you think you will need. You may want to buy art, which is a wonderful bargain in Cuba and it is exempt from the US embargo rules, meaning that it is legal to bring it into the United States and you may spend as much money as you wish on it.
You will want money to go out at night, and if you meet Cubans who would like to go out and eat, you will need to treat them because they live on very little money.
Normally, we suggest that people take as much as $300 USD per day per person.