Holy hell, there's way too much going on in NYC for free or low cost during the summer. Just sticking to movies, here's a few links:
River Flicks (look on the LHS for the link to the movie schedule) - FREE
Rooftop Films - $
Movies on the Hudson - Riverside Park South - FREE
Movies with a View @ Brooklyn Bridge Park - FREE

Don't forget there's also Shakespeare in the park, Shakespheare in the other park, Summerstage, Philharmonic in the park, kayaking on the Hudson, etc. ad infinitum. Looking at other cities offerings and we're blessed like a mofo.