Fortunately, some types of apps can get very nice revenue from ads. (I mean, compared to games.)

We're getting like $2000 a month out of ads with our free apps, but only $250 from our paid application, which is just a generalization of the free ones.

An interesting fact: we experienced no decline at all in the number of sold instances of our paid application when we increased the price from €0.50 to €1.00. That means, we doubled the revenue :-O. (Yeah, well, the new release included some improvements, but still surprising IMO.)