I suspect people are used to buying app-store apps at those prices because it requires no commitment. You could spend 99c on an app, use it just once, and not really regret it because it was "just" 99c. However if it is more than $10 then you have to actually consider the app's relative value and people aren't used to that kind of value proposition on the app store.
"No commitment" on the buyer's side, that is. There's still a high expectation of long term support on the seller's side regardless of whether the app has a very low price. Remember how many people freaked out over the sale of Sparrow? IIRC, that app only cost a few bucks.
They're paying for the market validation that can be eaten up by a company in a less expensive market to produce a knock-off version (or, more likely, ten knockoff versions) for that USD9.99 and enjoy boatloads of sales.
This pattern sets an expectation that, because it can run on your phone, it should cost significantly less than desktop software, even if it is more useful than a desktop version.
I'm a big fan of leverage and my father taught me the ROI model of tool selection from an early age, for everything from shoes to calculators. Unless it's completely unaffordable, the cost of a tool is irrelevant outside the context of its ability to amplify your thought and your output.