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.

[joe.attaboy]

What you are proposing is very common in the Android development community. I run a number of "lite" or free apps that have some kind of ad revenue addition. None of them are annoying at all, and although some people complain about these things, they're the same ones who complain about everything.

[Muhammad]

yes i agree with him, Android ads is not annoying at all like iPhone