According to Wikipedia it is 3mg, but it is not clear to which panel (treble, mid or bass) that figure refers. https://en.wikipedia.org/wiki/Quad_E...ic_Loudspeaker
My quoted figure of 100mg was from a reference that I now cannot find. The only other references state the mass of the diaphragm is very similar to the mass of the air to which it couples - but that is somewhat ambigious.
Whatever the true figure is, it will most certainly be less than 1g.