Prices at a resort are likely to be in line with prices in the US, but chances are you will be at most a 10 minute walk from somewhere you can buy them at the local price.
Quality is definitely not the same, many people tell me that Dominican manufactured Marlboro Light cigarettes are a bit stronger when compared to the American ones, and my personal opinion is that quality control is not as strict, I have frequently seen what appear to be clumps of tar in the Tabacco which as you would expect does not taste nice and looks funny when the fire hits it.
This is not frequent, but I smoke very little (less than a pack a week) and have come across this several times.