"We don't know because we've never done this before, and we can only compare to the most similar work we've done, and any existing data."
Before I could say more, they I interrupted: "You have to know, it's your job, or you don't know what your doing," or something along those lines. It was the most heated I've felt. I know what I'd reply with today, but I was younger and more naive then.
> "or you don't know what your doing,"
That though was unprofessional, and should be addressed as such.
Or just offer over-inflated estimates since they won't accept something realistic anyway.