I think the price is $6 per day for the adult and $4.50 a day for a childs... and you add the 15% tip onto that. Nearly $50 for one adult for 7 days. We just each carried on a 6 pack of plastic bottles for about $4 per person. I really wouldn't want to try and drink $50 worth of soda in a week
The card price isn't so bad when you consider the $3 a glass / can on-board price. A drink with each meal would cover the cost of the card in one week at that rate.