Sure it's cheap, but for your money you get a falafel that falls apart before you start eating it. The sauce is too yogurty for me as well.
Across the street are two other places, Ali Baba and Yatagan Kebab that will make a better falafel for the same exact price, without the huge lines.
By St. Marks, Chickpea will make you a falafel on freshly baked pita, with twice as many falafel balls for $3.20.
I've eaten at Mamoun's a few times trying to figure out what makes it so much more special, but still have no idea. Anyone?