We'll need more information. Which port has the beach in question? Are you referring to an excursion with shoretrips.com? If so, please provide some details about it, such as a link or name of the tour.
It doesn't cost $70 to simply go to a beach, but if you looked at an organized shore excursion to a beach it can easily cost $70 because it has features such as included transportation, perhaps the use of extra-fee things at the beach, lunch, etc. It depends. Or you could just take a taxi to the beach and pay as you go for things.