It's interesting, ~20% now seems to be the norm. (Not just for developer platforms.) 10% is pretty reasonable. I also used to work for a few years through upwork, sill when it was elance. I kind of remember that the rate was around 10%, maybe higher for smaller projects. Again, pretty reasonable.

A few years ago I've co-founded a platform for remote psych counselling and I remember my partner was pushing for a 30% commission. I told him that's how you motivate your users to try to get around you and not keep paying in the long run.