the later in August you go, the more you'll save... there are some Carnival cruises that depart late August starting at $429 per person out of Galveston.

edit: as far as cruise lines go though, i'd stay away from Carnival and seek out Royal Carribean. We had fun on our cruise... or I did... the wife didn't care for it so much. All you do is eat, drink, lie around, and pay out the ass for excursions.