A full day of deep-sea fishing is going to run you about $450 US. Go to the home page and click on the fishing link for more info.
If that is too steep for you, I suggest finding someone to take you on a snorkeling/fishing/beach bbq tour. You will get a full day of reef fishing (barracuda, snapper, jacks, and maybe even some lobster!), snorkeling away from the crowds, and then they will cook your fresh catch up on the beach. It was one of the best meals of our entire trip, and a full day will run you around $200. I could have done this every day...it is a blast! We went with Jerry Ack and Roque Caliz at Sol Caribe. I know that Ched Cabral also does this trip (you can find him over at the Underwater Video shop across from Fido's).