Over the time Errant has been running (coming up 4 years in September), it's made me a total of a bit over $800. Nothing amazing, but a nice bit of money on the side. For reference, in that time, Errant has built up 2600 subs.
Most of my revenue comes from ink donations, especially around Inksgiving. Ad revenue makes me less than a dollar a month, and really won't be something a person can rely on to make serious money unless they're in the tens of thousands of subs. I also occasionally make a bit off the Creator Bonus Program, but not loads; Errant is unusual in that it was invited to the CBP at just 1600 or so subs, which is a very long way below the 50,000 subs required to apply for it! 
It's best to pair Tapas with a Patreon and also things like merch and print Kickstarters. I make about $30 a month off my Patreon to add to the Tapas earnings, and the first print book Kickstarter was successful, raising enough for the print run and making me a few hundred on top.
I'm not going to quit my day job any time soon, but it's doing okay. I think if a person wanted to make their full time living off Tapas, they'd need over 50k subs and also a patreon and other income streams as well to supplement it.