It will be cheap if you book a package to Varadero with Sunwing, Westjet or Air Transat. While you are in Varadero take the Viazul bus to Havana for a few days and book a hotel or airbnb(i dont encourage this). Because the packahe to Varadero will inlcude, hotel, flight and meals and it will be cheaper than booking a flight alone.
Book using selloffvacations or itravel2000