Let's just go with some fuzzy numbers
<$11 billion to develop a Hep C treatment
4 million with Hep C in the United States
50 million with Hep C worldwide
So, big picture here, let's assume that only 80% of the world's Hep C patients are prescribed and can access the drug Gilead bought which we're assuming was $11bn to develop. If we amortize that to 40million patients, the mathematical expression for computing the amount each patient would need to pay for Gilead to break even:
X (each patient's fair share of the amortized R&D cost) = $11,000,000,000 (The R&D cost) ÷ 40,000,000 (The number of patients)
So, each patient's fair share of the amortized R&D cost is $275
Let's remember, however, that not all of Gilead's $11bn acquisition can be fairly apportioned to its portfolio of Hep C treatments. So it's more accurate to say:
Each patient's fair share of the amortized R&D cost is <$275
Of course there are more costs involved when you are producing and distributing Hep C medication, on top of the R&D required for it to have existed in the first place. I don't think that number, per patient, is particularly high.
Whatever the true number is, it's beside the point. What these companies are doing is morally indistinguishable from extortion. It's indefensible.