I just found out that if you are a Captains Club Elite member, you get 15% off this upgrade cost. However, there will be an additional 20% added for gratuity, so it brings it back to about $17.4 per day. I was also told that one person can do the upgrade while the other person in the cabin doesn't (i.e. it does not require both persons to buy the upgrade).
After reviewing the list of drinks provided under the Classic package and the Premium package, I think it will be worthwhile for us to do the upgrade. Some of the cocktails we like cost more than $9, and we certainly will try some of the better wines, whiskey, beers, etc.