This drug will only work on, let's say 1500 people. Of those, let's optimistically say that 1/3 have insurance that will cover expensive treatment. So 500 patients can be expected to be treated by this drug per generation.
So this drug will optimistically get a revenue of $500 million per 2 decades or so, or $25 million per year.
The number generally thrown around for getting a new drug researched and onto the market is $200 million. So it will take 8 years just to break even (although likely front-loaded once it becomes known that this drug exists), and total revenue for this drug for the forseeable future is about 500 million.
Does this seem spectacularly unreasonable ?