I have a few games out on the Apple store as well as Google Play store. It's a low priority hobby for me, but I've messed around a little with Google Ads. Not sure if it's because my target CPA (cost per action) is set too low for Apple, but it helps on the Android side for sure and zero for iOS. I've spent $100 total (see, small time!!!), and that was free money Google credited me for signing up. So far, zero out of pocket. I also "wasted" a lot at first with too high of a CPA setting. You can actually go as low as $.01 or $.02 and still get ads clicked on, but in my experience if I bump up to $.05 target I'm able to get a lot more hits.
Good timing on the question BTW, I just decided a couple days ago to restart my ad campaigns using my own money this time. I'm limiting my self to $1 per day per app for my 2 apps I care about and $.05 CPA target. So far it looks like yesterday I got ~140 clicks for ~$1.80. Not sure how many of those turned into active installs, but it's better to look at this stuff on a monthly basis, instead of daily. My piddly little games with $100 in advertising back in July/August last year earns me a whopping $15 to $30 a month. Technically I'm in the black. :). I have a banner on the main screen, interstitial every few levels, and no reward videos (which I think is where the good money is).
My point in all this is, you can start for nearly free. If google is still giving away advertising credit when you sign up, then you can try it literally for free. Set your own targets, limit your budget, etc. Whether you make a lot of money or not really depends on how good your games are and how much people play them. One thing is certain, before I started advertising I had essential zero installs and zero income. Since you are probably going to ask, my games generate an average of around a half a cent per active install per day. Not much, but with a low enough acquisition cost and high enough retention it's profit. I'd do things differently if I was serious and trying to make a living, but for a hobby this is pretty much risk free.
edit: To be clear, while I started with Godot and did have games written in Godot on the app store, I re-wrote them in Unity due to the difficulty getting admob working for Android/iOS. Things have changed quite a bit for Godot since then, and my next game I'm getting ready to write will be Godot, even if it means no iOS or no AdMob.