Ask HN: What is the cost per install rates for free iPhone app?

[shibataism]

AdNetworks for apps such as AdMob and Inmobi provide CPC/CPM ads but I'm wondering how much does it cost to get a new user for free iPhone app?

Tapjoy offers $0.35+ per install but it seems too cheap. https://www.tapjoy.com/site/app_developers