I think we can all agree that it's reasonable to ask Mitchell Bard to support his citation, and you claim you have done this and he refuses to do so. The citation of Mitchell Bard should carry the caveat that he has been asked to support his claim and refused to do so. The date of the estimate is also relevant and should be noted in the text. If these are factual statements about which we can all agree, fine, otherwise there may be a bit more haggling to do.