I've no idea how much they make, I think one person said something about getting a check for £200 and they mail out checks every 3 months. But she's also one of the most popular people on DA.
A combo of very good art and promoting themselves, doing stuff like commenting a lot, going on the forums and chats, submitting their art to clubs, etc. You can be the best artist on there but completely unknown if you don't work to promote yourself because it's such a huge site.
I like creepy stuff 😛