Never actually said it out loud before. I know ee-eye is the proper pronunciation for species names, but I might get sloppy if I did say it. Then again, after seeing this thread, I'll be more careful about it.
Fun question. Post 5 above may be correct. Who is our forum's cunning (professional) linguist? I guess the pronunciation depends on the latin root of the word if based on English. Yet, is the word Prius based on Japanese language roots? FWIW I always thought it rhymed with "Wee" (OK, haha pee) so "Pree". Again good fun question.
Toyota's information about the Prius name answers the question: What do the different Toyota model names mean? Derived from the Latin prefix meaning "to go before" So the Latin plural is Prii (pronounced "pree-eye")