I'm running some test campaign on Admob, not much, budget 10$ a day. The question is, the CTR is pretty high, about 1.79%. Why? And the minimum bid is 0.01$/click. If you are advertising a 0.99$ app, it means that you have to get at least 3 downloads per 200 clicks to cover the expenses... I'm doubting the effect.
Why is CTR high? You should know. Maybe you have decent copy or maybe there is a lot of missclicks? CTR is nothing, what matters is install rate. Honestly, advertising freemium apps is much better. You have much bigger install rate and then you can monetize those downloads in your own way in a long-term run. For example, on Chartboost you can easily get $4 CPI and it's still worth it if you can monetize that traffic properly.
A brief campaign I ran through Chartboost had a click through rate of 10-15%. The install rate was pathetic however, barely 1%. Paying per click will rarely work out. Paying per install doesn't really work either (directly) - you basically need to pump enough money in till the game charts and gets organic downloads. That tends to be a lot of $$$ however.
