I have been building the app for many years now. In the beginning I spent money mostly on instagram and facebook ads. I had pretty decent results on this, and could get installs for around 20-30 cents. However, those users also trashed my app in comments and didn't convert to paying users. Later I moved to App Store ads, it costs a bit more for the downloads, but the users convert much better. I am starting to think of going back and giving IG ads a shot though.

Edit: forgot to mention, I also spent quite a bit of time improving app store ranking, which contributes significantly to getting more downloads and customers.