The refillable drink mugs cost $5 per day of the cruise, but you can only purchase it for the entire cruise. In other words, it costs the same if you buy it on the first day or the last day.
A 7 day cruise is $35, A 3 day is $15, and a 4 day is $20.
You can get a free refill of fountain soda drinks anywhere that they have them.
If you purchase a soda without the mug, it costs $1.50 plus the automatic 15% gratuity. So if you drink more than 3 sodas per day outside of meals, you would be saving money.
You also get to keep the 12 ounce mug at the end of your cruise as an additional souvenir.
You are not permitted to use your refillable mugs again on another cruise.
Currently, if you book your cruise with AAA, they provide you with the refillable mugs for free.