As Arment notes, there's no way you can build something unique unless 1. you've got dedicated hardware (in which case you'll sell the hardware and give away the application) or 2. it's a small niche market through which you can stay under the radar of price dumpers
But there are nearly infinite numbers of "small niche markets", many of which are willing to pay a lot of money because in their case the app provides enormous value.
There are a lot of small niche markets, but I am not convinced that they'll generate much revenue because--hey, it's still just an app, why does it cost so much?
If the only way to do anything in mobile is to constantly be outrunning the onrushing boulder of the shitty free knockoffs, it probably makes more sense to just go do something else. (This is where I'm at now; there are no good applications for a number of things I really want, but there's also no way to recoup the time investment of making one just so you can get one-star reviews for "$5 is too expensive".)
