From Freakonomics:
A report by New York City’s Metropolitan Transportation Authority seems to prove that hopping a subway turnstile is worth the risk of getting caught and fined. The MTA estimates that riders entered the subway without paying 18.5 million times in 2009 (an average of 50,684 a day) while the police issued just 120,000 summonses, or 1 for every 154 jumps.
The report figures that a regular turnstile jumper has a chance of getting caught only once every 6 to 13 weeks. At $100 per fine, this works out to be cheaper than a $27 weekly unlimited Metrocard that would cost $162 over six weeks. So the fare-skipper who gets nabbed only once in that period still comes out ahead by $62. And that was in 2009. While the price for a weekly pass has since increased to $29, the cost of the fine has not, so in 2011 it pays even more to hop the turnstile.
In no way do I endorse any criminal activity, including jumping a turnstile.