Well, you need to really be careful on iAD, as you must choose the right Nation which invest into.
For example...
- Italy -> 1500$ -> You get in the top 10
- USA -> 6000$ -> To get a good position in the top 20
So is very different from nation to nation.
Also there is the CPC BID trick and you need 2 hours to adjust and get some statistic about tweaking it.
For example...
If i start a campaign for a game and i set the CPC BID on 0.50$/1000 impressions, i must pay attention if there is another campaign that BID 0.55$ or more.
So i need to find the right BID to go over some competitors.
So if there is Angry Bird that offer 2.00$/1000 to get more impression i need to set my BID to 2.10$
This is the way Apple make the money go around in a great business.
This is why iAD is better than the other advertising systems.
If you need more info i can open a post on it, on how to promote your app on iOS.
Long life to Steve!