And no security searches and long queues that you would do if taking the tourist boat from Battery Park. You get a fine view of the Statue of Liberty and Manhatten skyline for free. The money you save can be used for something else.
Try to book the Top of the Rock for just before sunset if you want to get a nice setting sun over Manhattan view (assuming weather is good) . Boat trip round Manhatten is good but can get really busy in tourist season. Cheap option is take the free Statton island Ferry over and back. Nothing...