^Keep in mind it's also cheap then because it's hurricane season and you could get side tracked a bit. Make sure to leave some flexibility in flight plans!
You'll need about $75 a person for tips, plus I'd say another $150 - $200 per person depending on how much you drink, use internet, go to specialty restaurants, and do shore excursions.
We usually book with either the line themselves or a reputable cruise agent who can get you some on board credit to use.