In 2011 I had a discussion with an Android programmer who told me about a throw off but popular app he and others had written, and how much they had made with the ads they put on it (in my recollection it was hundreds of dollars at that point). That was motivating, but also put the idea in my head to go with ads, not pay apps. I made a little money in 2011, but in 2012 I started making better money. Admob ads only shows me back to September 2013 right now - from December 2013 to March 2014 I had a free book app that showed over $2000 a month in ads. It sometimes dipped but made over $2000 in January 2015 as well. I don't have access to all the revenue I made at the moment, but had a few apps reaching hundreds of dollars a month, whereas the book app could break $2000 a month.

I had an idea of all expenses coming from revenues, so other than the initial $25 to Google and my time, all expenses came out of revenues. Initially all apps were on-phone with no backend. Initially I showed ads but did not run ads to advertise my apps. Initially I used the emulator and not a real phone. As I made money I bought a used Android device on Ebay (less than $100 with shipping), paid $10 a month for a backend web site (paying more later) and started advertising my apps, especially new apps, or new markets for old ones. It would probably be hard to have a successful app without advertising it to kick it off nowadays.

In 2016 I started writing an app to get free wallpapers, which I wrote a series of blog posts about here - http://www.vartmp.com/dev/wallpapers.html (I have it on my todo list to make that old site https in the coming months, it's easy enough to do but low priority). A link to the app is on that page.

Actually there is a blog post to be written of August 3rd 2018 to spring 2019 of fixing up Kotlin code (which I was less familiar with then) and other things. Aside from watching the backend server stayed up I basically abandoned the app since spring 2019 since I was busy with work and did not advertise it - and that is when it finally started making money. It made $192 in September 2019 and over $240 a month from March to May 2020. Since August 2020 it has made over $50 each month without any ads or anything from my end, so it pays for its own backend hosting, domain name registration and so forth.

My day job is working on an Android app for a large company. Over one million dollars a day in revenue goes over the app for the company I work for.

I did my side apps while I was taking college classes and was not focused on a good-paying, full-time, non-contracting SWE job as much, which I am doing now. Aside from monitoring the backend server is up, I have basically abandoned my side apps since contracting and then being hired by my current company, as my TC dwarfs what I made on my side apps while taking more college courses (although making over $2000 a month on my side apps while taking college courses worked for me). The Wallpapers app I released over three years ago still brings in over $50 a month, with no effort at all (aside from the very minimal effort of monitoring its server is up) from me.

Insofar as advice - for making money I favor ad-based apps over paid apps. The barrier to adoption are lower, less worries about piracy, and also you may not have a success until your second or third or fourth (etc.) app - I program Android full time and still haven't fully wrapped my head around things with a stable release from last year like Hilt and Compose, so there will probably be a learning curve for people less experienced.

I would also advise on focusing on big markets over niche markets. There's more competition but it's harder to make money in most niche markets.