It's been a couple of years since I was on RCCL, but I think they were pretty much the same as others. Beer is roughly $6 to $8 depending on what you like to drink. But they usually have a special whereby you buy a bucket of 4 and get the 5th for free. Wine is roughly $8 to $12.
Figure about the same as a regular bar.
But don't forget, you can bring two bottles (750ml) of wine and/or champagne per stateroom onboard with you in your carry-on bags at the initial embarkation port only.