what's your prior for my statement being true of a randomly chosen person

Sufficiently close to zero.

what's your prior for a randomly chosen statement I make about my preferences being true

Depends on the meaning of "true". In the meaning of "you believe that at the moment", my prior is fairly high -- that is, I don't think you're playing games here. In the meaning of "you will choose that when you will actually have to choose" my prior is noticeably lower -- I'm not willing to assume your picture of yourself is correct.