I am a diver. I went through CCAC to get my certification. They said it would be ~140 per person but all said and done it was 700-800 bucks. And I may be underestimating it.
At a resort you don't get a certification, Naui or Padi. They just certify you to dive that day with them.
That being said, it was worth it. It is a alien world yours to discover down there.