This is a great question. It's important to note that ATM, debit, and credit cards from the U.S. do not work in Cuba due to the U.S. embargo against Cuba. Therefore, any spending money you may want or need for incidentals has to be brought with you in cash. How much an individual spends when traveling varies a great deal, depending on personal preference. We do recommend bringing a minimum of $100 per day. If you think you might want to purchase artwork, music, cigars, rum, enjoy evening entertainment, or buy gifts for friends and relatives, you may want to plan on bringing more than $100 per day. It's always easier to bring your excess money home with you rather than to run out of money while in Cuba.
It's also worth noting that Cuba isn't a commercial culture, meaning there aren't a lot of items to buy. However, items that are available are generally expensive. Most prices for goods are comparable with costs in the United States.
Due to President Obama's December 17, 2014 announcement to normalize relations with Cuba, Americans are now allowed to bring back up to $400 worth of Cuban goods of any kind, with a maximum of $100 of cigar or alcohol purchases. The $400 limitation does not include artwork, music, or informational materials, which are allowed in unlimited quantities.