PhD candidates are paid salary in the Netherlands. Less of course than the senior trial researchers at pharma. I also would not assume there is no compensation for subjects as thats part of the medical-ethical process and not just ‘it helps the subject so no money’.

The €30k to €35k per year they make in this context approximates to free. Any normal company has to pay much more than that for less. A non-phd software engineer will get paid much more to change button colors for example. So that can't be the reason they are expensive and a PhD candidate is doing many other things than just helping to run trials.