we actually just did that cruise in april... it was a quick get away for us and another couple. i think our sign and sail was $250, but i think we each had one or two drinks the entire time, spent about $30 total on souvenirs for the kids onboard, and bought a few pics - not sure what else - i'd hate to see a bill with 10 drinks a day(but, we intentionally didn't drink much for that reason, i'd rather not pay $15 + 20% gratuity, i can do without it)! we prepaid our excursion(paradise isle i think it was -i would HIGHLY recommend it, free drinks and food there as well!) and our tips - so that doesn't figure into the $250.
if you do an excursion, 10 drinks a day, get a few pics, and tips - i'd say plan on $800-$1000 for your sign and sail, prob closer to the $1000.