I'm not a physician and not in the medical field, but I would hazard a guess that a lot of the expense comes from just doing the work. What specific doctor will administer the vitamin C and monitor the patients? How do you isolate that the vitamin C dosage increase is effective? Who is going to create the vitamin C in the proper dosages? Who is going to write about it to make sure that it's legally approved? The human body is very sophisticated. The trials have to be done in a scientific way, following the established procedures of ethical medical treatment, peer reviewed, etc. And let's say you start giving vitamin C to some of these patients and they start having bad reactions and it makes their disease worse? Who covers the hospital stay? Who pays for their care?

Just looking at a few things there I'm guessing that's a few million dollars at the very least.. and even so you have to look at opportunity cost. Is this the best and most promising path of research for the physicians and researchers? Are there more promising compounds? Etc.