The problem with targeting a niche market of developers is that you have to overcome the "I could just build this myself" syndrome - whether that's through pricing, ease-of-use, etc.
I'm, admittedly, a miser when it comes to paying for apps - I've only bought a small few. Consequently, I weigh the decision to drop $10 on an app pretty heavily.
Beyond that, I have very much a "learner" personality. I've created many an app just to play around with something new. I've yet to build anything that interacts with the Heroku API, so creating an app that does so allures me. The joy of learning and experimenting outweighs what my time is theoretically worth.
Granted, this is just how I see things. Others probably have very different valuations of the app vs their time. I agree that my previous argument isn't really valid from a purely "I'll build it so I don't have to pay for it" perspective.
