It's just three blokes and a bunch of contractors. According to Wikipedia Hollow Knight sold more than 15 million copies, at $15 that's $225M. Of course you have store fees, and discounts, and taxes, and whatnot, but all of that accounted for they still made at least dozens of millions – if not more.
It's unclear if they would make more money if it was much more expensive. I picked it up for €19.50 today; not sure I would have paid €80.
No wonder Silksong is only $20. I saw the memes about the devs asking for $20 and assumed it was $20 more like $60=>$80 or something like that.