…has there been a generic priced at "only" a 15% discount to the branded?
Yes, this often happens during a generic launch’s 180-day Hatch-Waxman exclusivity period.
Moreover, there have been cases where a sole generic persisted for years with only a slight discount to the branded product. I’ll make the identity of such products a quiz :- )
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”