I've been using AdMob, and recently added PubCenter to my ad rotator. To be honest, ad earnings have been very low for me. All combined, I still can't pay 1 hour of my working hours, let alone the 100's of hours that actually went in making these apps. For AdMob, the 'Request RPM' lies at 0.07Euro (average from 100k+ impressions). For PubCenter, the 'eCPM' lies at 0.16Euro (average over 10k+ impressions). I've moved 80% of my ad impressions to adduplex for this reason (I'd rather have some advertising for the app instead of the marginal income from the ads).
Main contribution to these stats is the
Cover - Facebook edition app which is ad supported but has the option to remove the ads via IAP. The revenue of this IAP is slightly higher than that of the ads (though still very limited - approx 0.2% IAP purchases / download).
As I said, with all combined earnings I can't even pay a single hour of my normal wage from building these apps. Luckily I'm doing this as a hobby
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I'd be happy to hear the experience from other devs!