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.
Yes, shore excursions are quite expensive depending on the distance and what you will be doing, what all is included, etc. You can find some independent shore tour operators, just do some homework and make sure the time limits work with the cruise schedule, you don't want to miss the ship.