In all honesty, cruises out of NY and Boston are rarely discounted. In fact, they are almost always priced higher than anything you can find out of Florida, simply because of the demand. There are so few ships here vs. in Florida that they fill without the need for discounting.
The key to a low price out of those cities is booking FAR in advance. The Carnival Cruise we are going on out of Baltimore this August was booked in May of 2009, almost a year and a half early. Prices have increased $200pp since we booked.