That may be true about the Apple App Store. Also that there is no control to check.
But you still haven't provided an argument that justifies the triple price.
After all, you didn't have to reinvent the wheel because you were able to use a large part of Tweetbot and the experience gained.
So why is the price three times higher?
It can't be hosting and energy costs alone, because you already had these with Tweetbot.
@Th3G4m3r TB was extremely underpriced. Intentionally because of how bad the public API was. Moving Tweetbot over to work with Ivory was no easy task. It's taken us 3-4 months of working 12-14 hour days just to get to this point and we still have years of hard work ahead of us to get the app where we want it to be.
$15 a year is not a lot of money for quality software that you use everyday. If a user thinks it's too expensive, then they don't see the value of the work we do and that's fine.