AFAIK, there is no in-game microtransactions or anything, so basically retail sales I suppose? The studio is relatively small (indie-scale) last time I checked, and the launch was pretty big and I'm sure they pulled in a ton of money, even though as always on the "vocal internet" the launch was a huge failure, they still sold a lot of copies. "Just" 1-2 million copies would go a long way to fund a small passionate team. The all-time peak for simultaneous players on Steam (PC only) was 200K, so I don't think 1-2 million copies sold is far off as a minimum.