The cost is driven down, because in 1 campaign you have 10 ads, and when you run it each add will be given a chance to get some impressions. The key though to getting the cheap clicks is having the daily budget at $10 and your cpc between $0.20-$0.30 because you'll be outbidding quite a few people and with a decent ctr you just get so much traffic so fast, you get this huge overrun of traffic before admob has a chance to stop the traffic coming to your ads. Before you were able to just run to $10, they'd pause it, then you'd up your daily budget by another $10 and just rinse and repeat.