The most interesting thing here was how much cheaper AdMob was. If the conversion rate for AdMob is roughly the same as iAd then the cost per acquisition is $2.43.
That's still not good enough for a $1 app, but it's a lot better than $14.90.
AdMob/Google likely have better targeting algorithms due to amazing amount of data that they posses. Also Google employs army of Machine Learning/Recommendation System engineers, while Apple is just getting in to this field.
My theory is that the CPA was high because iAd is a new and captivating ad-platform.
Users who see AdMob (or other older-platform) ads are less likely to click on ads.
However, most users haven't seen iAds and so they may click on the ad just to explore it further - this means that a lot of users may click on iAds even though they may have no interest in the product being advertised.
That's still not good enough for a $1 app, but it's a lot better than $14.90.