Check out the website
www.skyauction.com for different cruise line offers, I got a round trip ticket to Australia two years ago during Christmas (their summer) for like $600 when most airlines were charging around $2000...you can get really lucky with some of the auctions, so I'd assume that cruises would be as good also. I'm currently looking there for Spring Break cruises too
And they have a lot of different cruise lines, so you can probably even find a good deal on the one you want.