I know more about nonhuman studies than clinical, but according to the US HHS (who runs FDA), the breakdown of costs are these:
- $15k/patient for phase I
- $20k/patient for phase 2
- $25k/patient for phases 3 and 4
The cost of the average trial:
- phase 1: $4 million
- phase 2: $13 million
- phase 3: $20 million
- phase 4: $20 million
Some phase 3 trials can be larger and last longer than average, like 20,000 patients over 5 years. Obviously at the average cost of $25k/patient, such a trial would cost $500 million, well over the average. In fact, a long study can greatly increase the per patient cost as well.
Because multiple trials are run in each phase for each drug, these trial costs are multiplied.
The principal cost in any trial are the medical procedures (~25%): drug administration, tests (lab, imaging, biopsy, etc), exams, etc. These are repeated multiple times on each patient during each trial to monitor changes in both efficacy and safety.
Here's a thorough accounting from US HHS:
These costs are set by FDA regulatory standards and the medical laws of each country where the trial is performed. Of course if you want approval for your drug in another country, you must comply with all their rules as well, often repeating studies using their residents (e.g. Japan).
This 2012 Forbes article by Avik Roy offers further insight on why clinical trial costs are rising:
Pharmas must play by these rules, but they don't write them. Lawmakers do that.