How big is the RSS reader market? If you could sell your app to every single person who subscribes to RSS feeds that also uses an iPhone, how many sales is that?

My hunch is it's not a huge market, comparatively speaking. I sell public transit apps, which although has many more competitors, has, I'm guessing, way more potential customers. I've had a healthy full-time income from these apps for a few years now.

One thing that's in my favour though is you're unlikely to use multiple feed readers, but many people have multiple public transit apps.