Well first of all what I would recommend you is not to worry about the monetization side of things but worry about building a great app. Worry about getting users on board and keeping them happy. You can't monetize without having any users. I noticed most people have 10,000 impressions a day and are worrying about monetizing that. For Mopub the best thing to do is just contact them. I'm sure the guys at Mopub will be more than willing enough to help you out.