A couple of dollars is actually pretty expensive for a mobile app - people are conditioned to pay 99 cents at most on iOS for apps these days. Even a $16 app by a noted developer is often marked as "too expensive" on reviews (look up Square Enix's apps) - so apps costing more than 99 cents tend to have to work hard for it.
If you look at the iTunes reviews, you'll find TPA consistently reviewed as "expensive" (which it is for a mobile app alone). I'm surprised the Mac version is only $10 (for all 4 initial tables) - that's probably the "cheap" side for a computer game. Heck, I'm planning on getting it for the Xbox where it's the same price. While convenient, it does assume everyone is willing ot pay the same price for the various platforms, which isn't correct - mobile users demand cheap apps, while I'm sure they could charge $20 quite reasonable for PC and Mac, and 10-15 on consoles.
Of course, the biggest thing I'd like is some QA on it - making sure the existing tables at least work (ahem, MM no playfield lights).
Compare TPA to most mobile apps...
Most of the 99 cent apps are barely worth the 99 cents.