If I remember correctly, we paid about $450 each for airplane tickets in Semana Santa this year, but I checked a while ago for Semana Santa next year and it had gone up to around $600, which is crazy considering it's so close.
I would recommend the French side of the island to food lovers because of the many wonderful French restaurants and cafes, particularly the town of Grand Case, which is a gourmet paradise. It's also good for people who enjoy sailing. There are several marinas from which you can either charter a boat or join a group to sail out to one of the nearby smaller islands and go snorkeling. If you are looking for beaches, there's nothing on St Martin itself that matches, say, Bavaro, but nearby there is the island of Anguilla, which has beautiful beaches. We had an excellent time, but if you are going there from the DR and it's not your annual "escape the cold and relax" holiday, then you won't need more than a few days there to explore what's on offer.
The hotels and restaurants are a LOT more expensive than in the DR. We couldn't find any budget places to stay.
Hope this helps.