No you are not allowed to take your own alcohol on board. Wine is quite expensive compared to UK prices, and as far as I can remember beer is about 5 dollars, but they have a great selection.Do not forget RCI, like most cruise lines, automatically charge a 15% service charge onto any bar bill. I went to the Caribbean with RCI last december, and as far as I recall the cocktail of the day was about 8 dollars. Have a look at RCI's website, I am sure exact costs are on there somwhere.
Yes it is more money, but with regards to wine, on the 1st day you board you are able to buy a wine package. They have 3 grades bronze - cheapest to gold the more expense, then you have a choice of how many bottles in the package 7, 10. They also usally give you for the 7 package a choice of 3 red, 3 white and 1 champagne. You can decided just to keep to the same type each time. Also if you don't finish the bottle you can either take it back to your room, or they can keep it for the next day, either for lunch or evening meal.