lindsay, thats a good price, it also depends on what comes with that price. Try this website: www.shoretrips.com, its pricey but comes with ferry ride, beach trip with all its supplies(chairs, umbrellas, snorkel gear, lunch, and the dolphin swim).
The other site if I remember correctly is : www. saint-martin-activities.com.
Good Luck, Paul
Swimming with the dolphins is worth every penny. I had a blast. You just have to make sure it is swimming with them as there was two kinds where we went. Swimming with the dolphins and the encounter. Encounter was just sitting on the side and the dolphins just coming up for you to touch. Swimming with them is GREAT.