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?
Step 1 - Shopify developers sign up to ConnectPlus (for free).
Step 2 - Shopify developers add a piece of code to their app to display other developers’ ads.
Step 3 - for each ad a Shopify developer displays, they’ll get a free slot in the ConnectPlus ad network to display an ad for their app.
Step 4 - Shopify developers can monitor their ad’s performance in real time and carry out A/B split testing.
I really recommended to check and try it. Moreover, your feedback is more than welcome.