Researchers used a special tool to monitor energy use by several apps on Android and Windows Phone handsets.
Findings suggested that in one case 75% of an app's energy consumption was spent on powering advertisements.
In the case of Angry Birds, research suggested that only 20% of the total energy consumption was used to actually play the game itself.
Of the rest, 45% is used finding out your location with which it can serve targeted advertising.
I'm not surprised that in-app advertising is an issue, but the 75% number should be taken as an edge case, say a calculator with an advertisement run over 3G. I do wonder how much data usage is impacted as well, for those using cellular connections. Anyways this is comparatively speaking, and done against other processing tasks, so on the whole it might not be that large since the screen itself takes plenty of power.