Free Concerts in NYC

My favorite band?  The Rolling Stones.  Cost of a ticket: Your actual arm... both legs and over $400.  For General admission.
Source: No Kidding; click away

Also, there is a giant list below of multiple concerts and venues with bands of all genres.  You know the penny that you didn't pick up on the subway plat?  That would be overpaying.  They are free.

Summer Stage - This New York City-based event will have over 100 free music, dance, film, comedy, family and theater programs in 17 parks across the five boroughs. It will go take place June 4–August 29, 2013