Licensed travelers, including family visits, are authorized to spend up to the State Department Travel per diem allowance, which was US$179 per day in Havana [1] and US$126 for the rest of Cuba at press time. Journalists may spend more than this allowance (the amount is unspecified) and other licensed travelers may spend additional money “for transactions directly related to the activities for which they received a license.”
Money may be spent only for purchases of items directly related to licensed travel such as hotel accommodations, meals, and goods personally used by the traveler in Cuba. Credit cards, including those issued by foreign firms, may not be used.
Accompanied baggage is limited to 44 pounds.
Regardless of the reason for travel, licensed travelers are not permitted to return to the United States with any Cuban purchases, other than informational material, which may be brought back without limitation (this includes art, CDs, films, etc.). The regulations apply even to foreigners in transit through U.S. airports: Since the U.S. has no transit entry, all passengers in transit, say, from Mexico to Europe, must pass through U.S. Immigration and Customs; any Cuban items may be confiscated, whether bought in Cuba or not.
Links:
[1] http://www.moon.com/destinations/cuba/havana