Their current game raised just shy of $100k on Kickstarter [0]. Presumably there have been additional presales via other platforms such as Steam as well. Given that, $10-20k as an art budget doesn't seem unreasonable to me, though I have no idea how to estimate how long it would take to do the art for one of his games and if that would be enough budget (probably not at $50/hour, but maybe at $25-30/hour).
$100K is more like $93K after Kickstarter's cut. Further, I can't speak to the tax implications though I would assume they're nontrivial. And he has to pay himself enough to live for a year or more to get the game done and out the door.
This stuff is really raw and really hard at the margins and tolerances we are talking about here.
[0] https://www.kickstarter.com/projects/619141553/queens-wish-t...