One thing that you need to take into account when deciding on a business model for your app are the costs that are involved and the actual profit that you will get every time someone buys your paid app or pays for an in-app purchase. This should be part of the research process that you do before developing an app.
This post will give you a complete picture of the transaction costs that are involved when you create an app for Google Play or iTunes. If you have a free app that only monetizes with ads, then this post will not apply to you, but you still may want to read it if you want to implement an upfront cost or in-app purchases in the future.
Google takes a flat 30% of all app sales and in-app purchases. If you have a paid app, be aware that you can change the price at any time, but if you change that price to free, you cannot start charging for it again. The only solution is to create another version and charge for it. This isn’t a transaction cost per se, but it can end up costing you time and money if you do not know about it.