Just an FYI - on formal night/Captain's reception it's "open bar" for like two hours or however long the reception lasts (at least it was when we were on the Fantasy - CM's were walking around with trays of beer, wine and that night's signature cocktail for the adults and ginger ale and Shirley Temples for the kids). Also, if you bring alcohol on board, whether you buy in port or bring from home, you will not be able to drink them while on the cruise. They will hold it for you and return it to you when you disembark on your last day. If you buy alcohol from the onboard shops, they will again hold it until disembarkation. They want you to buy their alcohol. So, in general domestic beers are about $5-6, imports are about $7-8, wine can run from $8-10 a glass/$30-50 a bottle (depending on the name, region, etc) and mixed cocktails run around $11-12. Some drinks might be higher, some might be lower but that's about it.
For a three day cruise, if you each have 2 beers (4 in total at $5.50 each) with dinner you might spend around $30 a night - including tax and tip (I can't remember if they taxed the alcohol on the cruise but I'm using Florida's 6.5% tax rate and an 18% tip). Of course if you drank more or had more expensive drinks it would cost more.
When you say that you cannot drink alcohol you bring on board, that restriction applies to non-beer or wine. Each adult can bring either 2 bottle of wine or 12 (I think) beers and drink them in your stateroom. You can also restock in any port you stop in.