Most developers with ads use an existing ad network that is VAST Compatible, like YuMe, OpenX, Liverail, etc. You add the vast playback code into your roku app, and they track what ads are viewed, by whom, however many times the ads play, and they pay you the money (a penny or so per ad that was played).
If you are setting up your own advertising network backend, in order to be competitive you would need to line up advertisers yourself and offer features and pricing comparable or better than what already exists.