Over the past year I've gotten between 500 and 1000 downloads per month on a very niche application that I built in December 2010. I haven't updated it or advertised it since a month after the initial launch. I want to believe it makes money, but I do not have any tracking installed whatsoever and cannot distinguish revenue made from the app and revenue made from its companion website. I would estimate it probably makes $100 - $200 a month, but that is really just a shot in the dark!