The refillable mugs are $5 per day for the length of the cruise, but it costs the full amount no matter what day you buy it on. In other words, if you're on a 7 day cruise, the mug costs $35 if you buy it on Saturday right after you board or on Wednesday or Friday.
You can get soda fountain drinks for free during meals in the sit down type restaurants. Some sodas are not available from the soda fountain, such as Diet Sprite. We have to pay the $1.50 plus the 15% gratuity for every can that we want, regardless if is during a meal or not.
There is another benefit to having a refillable mug if you would like a mixed drink with alcohol in it. They offer a mixed drink with added alcohol in your mug for $2.99 with several varieties, but I don't recall what they are right now.
hope that helps