it seems to work out with advertising, as it is for more and more app developers because you just get so much more installs without a pricetag on it. Most devs dont have any marketing muscle, so free apps with advertising often is a good approach.
You do the best you can with the resources you have to promote your app, even if its free, since that cant hurt. Even with free apps you still need a good app and luck, its not that any free app will generate tons of ad revenue.
