Do free Android apps have ads so the developer can still make money?

[ww2_honor]

I am planning on creating my first app for the Android Market with the Google App Inventor for Android (I'm trying to get in the Beta). I am thinking of many ideas but I know my first app will probably be a Twitter client. I don't want to charge for it so I can get good marketing to begin with and earn the trust of my users. I was wondering if you make a free app can you still make money off of them with the use of ads, like you can with the iPhone App Store which I don't want to develop for because it is so restricted. I will most likely make a Pro version of my app so users could pay something like $0.99 to get rid of ads and add a few extra features.
How much would you say you make in a month off of one of your free apps that use ads? If I can make enough I will most likely bring the app over to the iPhone. I use an iPod Touch and don't have a Android phone but I have access to one with a family members, which is how I will provide support for the Android version of my app.