Am I wrong or you should use Appodeal because it should automatically selects the best Ad network for your app?
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I start to think that the "In-app billings" let you earn more than advertising.
Most users do not take actions on advertising; to earn a few euro (or cents) thousands of impressions are needed.
By selling in-app objects, say for 1.99 €, you can earn as much as thousands of impressions, I suppose. Moreover, if then an user will no longer use your app, he will have already spent and you will have already earned. Instead, to earn from user's impressions he must often use your app.
For example, playing Zynga Poker you can send a donkey to an opponent; this donkey will exist only in that single game and you spend fake dollars (your capital) to send it.
I could instead sell the "donkey gun" as user's permanent object, for 2/3 euros (perhaps limiting its "firepower" to a maximum of 10 donkeys per game, to not clog the game).
I think this is more profitable.