It's not necessarily that they can't afford to buy apps, but that they live in an economy where credit/debit cards (and even bank accounts) aren't that common.

It's not necessarily that they can't afford to buy apps, but that they live in an economy where credit/debit cards (and even bank accounts) aren't that common. Most of Africa's like this, same with a lot of Asia - Visa and Mastercard have very little footprint there. Instead people use cash to buy pre-paid access from their mobile carrier. You're right there's a question around what kind of adverts you'd display to them though.