that pic used to be my avatar around here for a while
you pretty much summed up the main story with this beauty. as for being rare, i haven't seen many fs (and the last one i saw for sale was somewhere around the year 2000)
compared to a yakface, that makes it pretty rare.. as goes for much of the oddball items out there. that said, there's way less demand for oddball items, so the old question of demand and supply applies.
can't really put a number on it
i wouldn't mind owning one either
ps: i had no idea it was made from styrofoam! l'd like very much to see yours and the condition it's in, i suppose styrofoam is pretty prone to damage