Couple of recommendations to make the app a little more useful. Pretty much everything I'll suggest is a function of ManaBox, so that would be a good reference.
The most important things would be:
A sorting/grouping tool in the collection tab.
A proper folder function as the list function is somewhat inconvenient, mainly I'd like to be able to sort my main collection into folders.
A way to search specifically from collection while building decks
And a way to make decks register cards in your collection as being in that deck so you know how many you have available for other deck.
Given those updates, I'd absolutely pay upwards of 20$ for this app, but even in its current state it is extremely helpful, so thank you for taking the time to make it!