Think of a study that needs 3 lab test per day for each of the participants. These tests aren't cheap and a multi-year study with a couple of hundred participants with shipping costs (overnight) for the samples. It does ring up quite a bill.
Plus, oh, finding and keeping enough people in the study willing to get 1-3 blood draws a day???
Yow. I thought that by Phase III safety was pretty well established and you'd be looking for efficacy endpoints, surrogate or real. Emphasis on endpoint ... but I guess not.
But ... if vague memory serves, from the overview of some Phase III trials, maybe not the first one, I remember more like hundreds of participants. Maybe not quite a thousand, but a lot.
They don't have to be blood samples. I remember one which was blood, urine, and stool each day. Also, each sample might have multiple tests performed. Its a pretty interesting logistical problem.
Ah, that may explain the statistic I recently saw that Phase III trials are taking up 90% of the budget necessary to bring a new drug to market.