Maybe I was just lucky (though not that lucky), but it should be pretty easy to make back that $100 even without any marketing.
My opinion is that if your app does something slightly useful and basically works, you should be able to make back that $100 easily. Ideally, you'll make more than that especially over the course of a year.
Did you know that a lot of apps on the App Store are the sample projects from Apple themselves with just some images changed (almost all the level apps for instance)?
Some didn't even changed the images.
And, since the people approving the apps are a completely separated department, they couldn't know and they even featured some of them.
I don't know about the getting featured part but the apple sample code is provided for developers to use as they wish. I agree though that simply putting up the sample code is not very cool. There are not a huge amount of these as far as I can tell.
My opinion is that if your app does something slightly useful and basically works, you should be able to make back that $100 easily. Ideally, you'll make more than that especially over the course of a year.