The way to make big money in the cultural field is through IP. That's essentially what song writing, novel writing, art production etc is.
JKR will probably have made more money through selling the rights to Harry Potter than she will have made through the actual selling of the books themselves.
I knew a music producer in the mid noughties in a very niche field. He lived off just churning out hard techno, one after another. I also knew someone fairly famous in the field who made millions out of selling one particular tune to a company, and also getting publishing on every time the tune was played across global territories.
In the late nineties, it used to be, for example, that the BBC paid about £90 in publishing every time they played a song not on their A-playlist on the radio. It was more for A-playlist. I remember when I heard that; it made me realise just how much of license payers' money the BBC must have paid to artists like Madonna and Michael Jackson in the early nineties for any song they had a credit on (and they played Madonna and Michael Jackson on a loop pretty much back then).
It felt a bit immoral to me, tbh. I don't begrudge anyone being paid for their work, but when it's publicly funded and being so channelled to a select number of parties, it made me very uncomfortable.
It's also why Beyonce insists on changing a word in any song she performs (so she can claim a song-writing credit).