Try Santiago
Unless I'm mistaken Cubajet and Air Cubana are one and the same. I know I had to go through the Cubajet web site to book a flight on Air Cubana.
I think it now costs $450 from SD to Havana.
As I recall you can save more than 40% flying from SD to Santiago de Cuba insteead of Havana. The SD-Havana route flies Thurs and Sun, Santiago is Monday and Friday.