The time of year you cruise makes a difference in price too. As much as I love cruising in the spring, it's usually quite a bit more expensive than in the fall (hurricane season). I know lots of people won't cruise during hurricane season, but we've done so several times, and it worked out well for us. My rule of thumb is if I can get a 7 night for $600 (including port charges/taxes), it's not too bad. Have had them cheaper, and have had them more expensive. The thing to consider when booking in hurricane season is that you need to be flexible as your ports may change due to weather. (Actually, ports can change anytime for any reason the cruiseline deems necessary.)
Do lots of research, get some quotes (they do vary considerably). Other costs not included in the price: tips, alcohol, soda, bottled water, spa, casino, excursions, photos, oh, and don't forget SHOPPING! We've come back with a final bill as little as $300 including tips. We don't gamble, drink some, but not all day/night long. Probably a few per day, which is more than we drink at home. Figure $5-7 per drink.