Sail from the East coast for 7 days under $600 to Bermuda on NCL! Cheap,cheap!!! What gives?

One of the big reasons is they need to fill the ships. Cruising keeps growing every year. While still only about 11% of Americans have cruised. The cruise lines keep building bigger and bigger ships and they have more berths to fill. They offer the lower good deals and hope they make it up on the onboard spending-- Drinks, photos, spa, casino, shore excursions.
To give you an idea as of Feb 6, 2006 23 new ships were projected to come out between Apr 2006 and Fall 2009. Those ship represent 63,464 lower berths. I don't believe these figures include Royal Caribbeans "Genesis" project or Celebrity's "Soltice"

There was a story that circulated among the travel biz a few years ago. In it, a senior manager at Carnival was heard to tell a group at a conference that they could give the cabins away and still make a profit. I'm not sure if it really happened, but given the volatility of cruise prices I believe there is at least some truth to the story.

What time of year is this? If it's a May or September cruise, it makes sense. Yet, for the peak travel months of July and August (when school is out in the northeast), prices are considerably higher. (I know from experience...)

HAVE YOU ADDED ON THE COSTS ASSOCIATED WITH TAXES AND FEES? NCL seems to charge the highest out of New York!

Not all cruises are cheap it depends what "class" and line you go on