The soda cards are issued per person, it would be hard for the girls to share one at the same time though. The price usually runs around $5.75 per day plus gratuities. So if you are going on a 7 night cruise the price would be around $40.00 for the week. You have to drink alot of sodas to make it worth that price. There are "free" drinks available such as lemon aid, fruit punch, water, milk, iced tea and coffee.
Another alternative is to bring a few soft drinks with you. Carnival will allow you to bring a reasonable amount onto the ship, but if you try to bring cases of them on board they will confiscate them.