Out of curiosity, I dug around a bit. The closest thing I saw was that in June, Carnival has a 4-day and a 5-day cruise from New York to Halifax, that VacationsToGo is advertising for $379 and $399. You might get a wee bit knocked off of this from one of the other internet cruise sites. These are already steeply discounted, so they are unlikely to go down further closer to the date, though. The Carnival line and Halifax as a destination are both debatable, but for people who are easy-going and just want to be on a ship, they might be fine.

The other destination that is common from the East Coast is Bermuda, but all the cruises I see listed right now are 7 days and starting at $700-$800. I have seen shorter, cheaper cruises to there in the past, but nothing listed at this point in time.