I recently experimented with a free version of Touchcards called Touchcards Free. The free version runs the same code as Touchcards 1.1, but is limited to one flashcard deck. There are also ads courtesy of AdMob.
So far I've seen the free version get downloaded at about 20x the rate of the full version. The revenue from the free version in that time is about 20% of the full version revenue.
It's not a bad relationship so far, but I'm going off very limited data at the moment. update The numbers above have been updated based on 30 days of data and differ from the ones originally published.