That's the problem -- this situation is close to unique -- so everyone is using examples that are more normal.
Do YOU have any examples of a company with a single drug with an eroding exclusivity/patent profile and generic competition in the US, with no pipeline?
If not, you go to math -- it's the discounted PV of after-tax income. $1bn of sales for call it 10 years, after tax, and discounted at the cost of equity with a buffer for uncertainty? That's clearly between $1bn and $2bn.