I don't know if they make more than someone in a studio, but as far as freelancers making commissioned art, people who make adult stuff generally make substantially more. A lot of people don't do it because of A ) societal taboos or personal morality, and B ) they get asked regularly to make some stuff that would rend…
From what I gathered (uhhhh, "research") some of those people are hobbyists. They definitely used ripped assets from games and tweak them. But, from what I heard from other people, there are more serious hobbyists who have re-created entire game or CG models for those purposes. Speaking personally, I wouldn't judge someone…
I don't doubt that, but you can't really compare what artists earn working for individuals commissioning work for their own personal enjoyment (what you are describing) vs. a business paying you as part of an investment for a product intended to be delivered to mass market. Doing the former you aren't likely to even scrape…