In context of helping my continued path of studying japanese. What app would be the most helpful to download with ONLY $15 to spend in the iTunes App Store?

I currently have the

$10 RTK app
$25 Anki app
Imawa
$10 Midori app


If I had to pay for a service, what else am I missing?

This might give you some ideas: http://rtkwiki.koohii.com/wiki/IPhone_Applications

I have an Android phone, but the best paid Japanese-related app I got was EBPocket Professional, which is also available for iOS. It's seriously useful to have my EPWING dictionaries wherever I go. There's also a free, more feature-limited version.

Aozora Bunko formatted text readers come in handy if you can or plan on reading books in the future, and I've heard some good things about iBunkoS.

Edit: Double post removed.

Before I switched to Android, I found Midori to be very useful. Kabuto for Android just isn't quite the same to me