Flights from Cancun to Havana are U$328RT iirc. Visas are available at a kiosk in the Cancun airport terminal.
NYC to Cancun, you can price out yourself.

Insight Cuba seems to have some nice cultural tours ("legal" trips), starting iirc with a long weekend (4days/3 nights) for about U$2100.
http://insightcuba.com/