Android app marketing will allow you to advertise your own app inside of other apps! You can also make it so that other apps can advertise inside of your app. This can help make back some of the money you spent on making your app.
If you decide to put up an advertisement inside an app be prepared to spend a little. You should only spend money on advertising if you believe that ad will attract enough people that will spend money on your app, or downloads if your app is free with no in-app purchases. If you do decide to place an ad for your app inside of another app, you cannot just find an app that will let you and have them put an ad in. While going for the most popular ad allowing app might also seem like the answer, it is not always the solution. You should try to find a popular app that allows ads within the niche of your own app. For example: If you were a calculator you should advertise in an app that helps people with math.
If you plan on allow other apps to advertise themselves within your app there are a few things to consider. How much to charge is one of the obvious ones, and it entirely up to you. What to allow is important to consider. Using the calculator app example again; you should allow apps that are not in the same niche as yours to be advertised in your app if they want to be, you should allow apps that are within your niche also, you should not allow an app that could hurt your apps downloads (another calculator or similar app).
Complete list of Android App Marketing Lessons