Sorry. I wasnt super-clear in explanation.
Was assuming a total market potential of $1B per year, but assumed the successful drug would only capture 50% of that market averaged over 16 years.
So $500M per year actual sales over 16 years gives $8B total which is just enough to cover costs. No profit.
Means you need $1B per year potential to even think about a program.
(Which, as you stated, economocally limits targets.)