If I'm in Miami, what is the cheapest way to get to Cuba? Also, if I'm in Cancún what's the cheapest way to get to Cuba?
Be aware that for the most part, you can't go from the US to Cuba, unless specifically authorized by the government. Most people go to Canada or Mexico and fly from there.
As for what is the cheapest, that's hard to answer as airlines change their prices and can't be answered objectively and accurately as they're subject to change. In addition, it’s not possible to search for flights on large booking engines (such as Kayak) due to United States travel restrictions. As a result, you need to be creative: use European booking engines or go directly to the airlines’ websites. On a large European booking engine (Skyscanner) you can find flights from Cancun to Havana on combinations of Cubana and/or AeroMexico flights,. The flight time varies from 1 hour 15 min to 13 hours, depending on connections.