We'd do it in a heartbeat but are already booked on Carnival Sunshine after the transformation.
As for airfare, if you have enough frequent flier miles, some airlines allow for 1 way flights without charging you the RT award ticket price.
For Sunshine we are going over on United (Lufthansa) and coming back on American (British Airways).
But do check the cruise line. For crossings, they are often a deal.
OTOH, watch out for luggage fees - they can be pricy.