-Google charges 30% taxes as a comission to each purchase.
-It requires an android-seller account.
-Antipiracy systems, though not difficult to implement, are useless.
-Doubtful benefit. I don't really know how many users would be interested into this ad-free version, since only 2 asked for it. It would be nonsense to charge big money for it, so let's say 2$ would be reasonable. This means I'd getting 1.3$ per purchase.
Sincerely, check for appleads (i.e. Pontiflex) benefits on the web and you'll understand me.
Moreover, users who'd paid for the app would ask for immediate updates, better interface and more options to "Ny subway map". And I'm unwilling to face it. This app was created as a tutorial and a test for myself and a friend who was learning android. I'm not interested in spending dozens of hours in it.