It all depends on where you live, let's say for example that in some cities you can find travel agencies that will use you as a ''mule'' to carry on their luggage and will pay for your flight ticket.
Put it like this...
They charge Cubans residents (for the most part) living in U.S a certain amount of money to send their packages to their relatives in Cuba, and then, they need someone who will be delivering or moving them to Cuba, and that could be you !
In Las Vegas for an instance, I know of couple of companies that you only pay them like $200.00 and they allow you 20 lbs of personal belongings, the rest is what you gonna carry for them, usually 80 lbs, once you get to the airport in Cuba, they paid for the extra or excess of luggage, and someone will be waiting for you to receive those packages and deliver it to families whose relatives from U.S sent them, did you get it ?
Hope so, Good luck !