I think you cannot compare AppStore and iPhone with AndroidMarket and Android phones...they are really really different. But it can be a very interesting topic to talk about.
A nice paradigm you can use is to put a lite/free app to let your future users download it and try it and put a pro/paid version of the same app but with more features.
The evaluation is nice, but I think it's not what you were looking about.
I haven't still put paid apps on, but some of my friends did.