That's a 12% return in 2016, which isn't nearly as good as the comments in this thread would have you believe. It's an okay return, but it wouldn't really be a good investment unless they can grow those sales more. It looks like they have had some growth compared to last year, so maybe it will end up working out for them. It's certainly not "100x in profits".
A Google search shows the disease is ultimately hard-to-transmit and preventable[2]. $84,000 seems like a fair price to me for this particular case. Google says the total cost of raising a child is "$245,340", so paying a 30% premium in rare cases doesn't seem unreasonable.
A separate complaint is that the financial industry isn't covering this case well. If you expected your insurance to cover it and they don't, that is an entirely separate issue than the price of the drug. Even if they don't, it seems like there should be a market for people wanting loans to pay for cures: It seems really hard to underwrite such a loan, but it could probably be done.
What does good insurance from a reputable insurance provider that reliably pays out their claims in cases like these cost?
(Edit) I may have misread the earnings report, thinking it was for the previous year rather than the previous quarter. So the percentage is closer to 50%, and a commenter clarified that they also had another product.
[1] Earnings:
http://investors.gilead.com/phoenix.zhtml?c=69964&p=irol-ear...
Purchase price:
http://investors.gilead.com/phoenix.zhtml?c=69964&p=irol-new...
[2] Going down the list on one site that lists transmission methods: Don't have sex with weirdos carrying diseases; don't share needles with druggies; go to a licensed upscale commercial tattoo parlor instead of some shady cheap one