I liked the first comment (in the post) by Andrew Maine. Basically, it's simple math. If you expect a 5% conversion on a $1.99 app (which will earn you $1.39 after Apple's cut), the most you can spend for a viable business model is 6 or 7 cents per click.

Less if you want to make an actual profit on the app and not just get more users. But then, if you want more users, maybe the app should be free?