1. How do you get your clients?
2. Could you elaborate on "5-10k a year is much more common for niche apps with no marketing (what everyone seems to write)." I'm curious about your apps and results.
2. I don't talk about clients' numbers for obvious reasons. I'm not really sure how to elaborate. Can you elaborate on your question?
The figure I gave above is for a generic niche app that does a useful computation for those in a particular profession with no marketing budget. There are a lot of people who say to themselves "I'm a doctor, and I want an app that does X" where X is something doctors do. That is, the primary audience is the client. In cases like this, the marketing budget is 0 and the client's focus is to make a little extra income, but his day job is being a doctor, and making his own life easier. If he makes his investment back in a year, he's happy, and has 3-4 years or however long the lifecycle is of pure profit.
Other times you have people who use iPhone apps as a loss-leader (online ordering, etc.) for their real business. I have no way of tracking the "success" of this, but anecdotally the proposals are like 25% of the market.
The "startup" case--where somebody quits his job to develop the killer iPhone app--is actually not that prevalent (except on HN). 90% of these people are those flakey guys who "have a killer idea" that reduces to "Facebook for X" or "Let's make a CSI-enchance button for the iPhone camera." There are other problems too, that basically reduce to not having thought things through or not knowing what they're doing. The remaining 10% are legit, but it's work to separate them from the flakey people, and meanwhile I've got unsurmountable piles of Type A and Type B people's proposals in my inbox.