In my post The Cost of Knowledge from a long time ago I did not know the profits of De Gruyter but just made a guess by comparing the prices they charge.
In an interview with Sven Fund by Richard Poynder Sven Fund answered the question about De Gruyters profits. The answer was that everything is known and transparent. After a clarification question of mine we now have a pointer to the business reports of De Gruyter:
I would be glad if somebody could analyze the business reports of the past years and provide a summary here. Maybe this information should also be part of their Wikipedia page. It is for Elsevier and Springer, but not for De Gruyter.
The full interview can be found here: