Pheace: ... 10% after expenses or 10% total? Because payment services alone are going to take a couple percentage points of that probably.
The exact percentage is not as important as the idea that it should be way below 30, as much as possible.
If I remember correctly from CDP financial reports, there are about 150 GOGlings and if these are paid from 30% of the total revenue of games sold through GOG then we, the consumers who keep GOG alive, only feed about 350 additional devs and publishers (the remaining 70%) and who knows how many of them are actually programmers or artists. 30% just for presenting the game on a web page, processing the payment and offering servers for download seems like it's inefficient and should be cut significantly.
It may be that the amount of business that GOG could attract one way or another was just too small and the business model doesn't scale favorably for them. In that case they may be doomed and not able to compete with bigger businesses in the long run. Fixed costs may burden them.
While we are in an age where digital download stores pop up like mushrooms, maybe independent devs and publishers could form a business which they then own collectively and create their own store which operates at only 10% (including payment transaction fees) overhead. (www.dos.com - digital outlet store)