I have several apps on the iOS app store. I make between $4000AUD - $9000AUD per month.
I quit my job 3 years ago to do it, so I guess it isn't really a side project any more. That said, it is a lonely way to make money and I am over it. My career was computers, and I'm pleased I've done the apps, I can achieve anything technical so it isn't a challenge any more and am looking at something more physical, hands on.
When I quit my job I was making approx $3 a day from apps, I learned a lot.
The great thing is that I can quit and the apps keep making money. It is trivial to keep them updated to the latest iOS version, design standard, or advertising API - I've just put Firebase Analytics and the latest Admob API in them all. Pleased with the iOS10 announcement today, it will not add burden to my apps, nor are they sherlocked like my first app was. My most recent app took me 10 days to make, it is fairly professional, I have one more I want to make it will take a similar amount of time. Every time I get asked a support question I answer it and also post the answer on a support website, this has cut down support enquiries to one every week or two.
On to the next thing!