TBH the profit margin on this game is probably closer to 100% than 10%, it was a solo-dev game so never much overhead, I think one guy was hired to work on it.
Valve's 30% cut would lower it substantially. Taxes might put it beneath 50%.
The game has also been on sale numerous times for less than $15. It is currently available on Steam for $9
He also put an insane amount of effort, far more than most of us mortals.
All the game markets take huge cuts. Steam is 30% if I recall correctly.
Well he made a 10% deal with a publisher iirc and steam gets 30% - so that's a chunk right there
Steam takes 30%, other stores take 10-20%
Presumably he spent years working on it. His own time should be subtracted from the revenue too.
30% off the top for most stores (Valve/Steam, Apple/iPhone, Google/Android, etc), then around 50% taxes between state, local... some fixed expenses and overhead. It's probably well under 20% in the bank after all is said and done. That said, it's still a lot of money.