If you want to make money in software, research is key.
Experimentation without research is an easy way to burn through cash/time needlessly, and expensively, ultimately lowering overall ROI.
Revenue from ad placement is a function of views, and views are a function of both downloads and retention. Most downloaded apps live on a phone for just a few minutes before being evaluated and uninstalled, so retention is a more important number than just downloads.
Retention is all about product-market fit: how well does your app absolutely nail the user need? What is the benefit of your offering over how the user satisfies that need today? What is the differentiator that makes your app a better choice than alternatives?
There are some good talks on measuring product-market fit in the MicroConf series on YouTube.
Once you have confidence in your product, the rest of the ad placement game is about numbers. Big numbers. Not unachievable, but not likely to just fall from the sky for free. If marketing were easy everyone would be a millionaire, and inflation would be even worse than it is.
Marketing need not require much cash investment, but with or without much budget it will require time. Time dedicated to growing a business is a leading indicator of success. Spending time cultivating your business won't guarantee first-year breakeven, but not spending sufficient time will guarantee failure. Watering a garden may not prevent a snow storm, but not watering ensures the snow storm won't matter.
Reading up on "guerilla marketing" can point you to low-cost ways to spread the word about your product. And in the modern world social media is an important part of that breakfast, so plan accordingly. A basic marketing plan can be executed in just a few hours a week. But they must be invested consistently every week.
In most cases, when an entrepreneur follows this ordinary path to profitability, the low ROI of ad placement becomes self-evident, esp. when compared to the much higher returns achievable with nearly any useful SaaS offering.
But at a minimum, this journey will illuminate many opportunities along the way, perhaps even prompting a pivot from the app you started making to the app you discover a greater need for.
I believe there's no greater adventure available to anyone with sufficient motivation than making software in the 21st century. Done well, it lets you place what are effectively money printing machines around the internet, passive income streams that have the potential to beat most other side hustle opportunities.
It all begins with a great product that offers a compelling benefit easily communicated.
Find your microniche. Know everything that customer needs. Deliver that. Then plan your course of action for letting people know about it.
When those two steps are done, let's meet back here to discuss whether the low ROI of ad placement is even still interesting.