Yes, this is the question we are dealing with every single day (I’m a team member of several Shopify apps). As advertising apps is expensive (spending money doesn’t always guarantee results), we created and launched in-app ad exchange platform for Shopify developers. ConnectPlus - it's free banner exchange platform (even no credit card details needed) where you put script to your app and show other Shopify developer banners.
So, how does it work?
I really recommended to check and try it. Moreover, your feedback is more than welcome.