All drinks on RCCI include a 15% tip (automatically added) and a beer is around $5 - $6 inclusive. RCCI have a policy that all duty free alcohol must be left with them and is gicen back on the last night of the cruise. I have known people put gin or vodka into a water bottle and sneak it back on. The coctails range between $6 and $10. Hope this helps.
I agree with most of the previous answer, but do not attempt to put vodka in water bottles, RC often test water bottles to avoid this. This is to ensure that kids do not get hold of alacohol on board.