In my experience, it comes down to individual barns and trainers. There's no correlation between how well-known or accomplished the trainer is, and what you get as a working student.
I spent a summer as a working student in the States. The trainer was fairly well known locally, and he did all right, but we weren't talking big time. Nor did he have anything going at FEI levels, so he was hardly Steffan Peters or even on Steffan's planet. I got free board for my horse, free lessons, and free accommodation. I did not get a wage on top of that, so I was still relying on parents to send money so I could buy things like food. Such luxuries!
Another place I looked at offered all of the above, plus a stipend. Unfortunately their yard had no room at the inn for my horse, so I didn't go there. He was also a fairly high up trainer in USDF circles, had coached the US Dressage team and rode to Grand Prix himself, so would have been a way better placement. Hence the yard being full up.
I reflected on the irony that a Big Name Trainer gave working students a wage alongside the free accomodation and free horse board, whereas the No Name Trainer did no such thing.