The FDA forbids charging for any medication or medical device that isn't approved so medical trials are free by definition, but the Affordable Care Act requires health insurers to cover routine cost of care incurred during qualified trials. In a Phase I trial under medical supervision, for example, might not cost the patient's insurance anything but a Phase 3 trial for chemotherapy might cost a ton just to deal with side effects and drug administration.

As all things healthcare, it's complicated.