Late to this conversation, sorry! As a professional cartographer I am currently making just enough. I am never without work, which is nice, and I contribute a decent amount of money to the fam. That being said, if my wife didn't have a decent job then I wouldn't be able to do this, and I also pick up or side art-related gigs from time to time to keep cash incoming. I have some layout jobs in the works, and I also recently landed my first published RPG writing credit too. But like others have mentioned, if you set a rate standardization and work with a minimum then you can do decently. If you've done enough to get "a name" out there, if you have unique style and people come to you FOR that style then you can charge more and you aren't bowing down to the whims of publishers who may want something you wouldn't want to put your name on.

If you can bring in enough work, you can do okay. Keep in mind, however, that most working artists make less than 10 grand a year. You have to bust ass hard to make that, too.