I agree it comes up often, but the original estimate of 100 or so may actually be accurate. If you figure that in contrast to C1/C2 or Garden Path, this item was obviously a collectible from the outset. C1/C2 were mostly used and thrown away after the tournament (it was the 1970's and no one was thinking of D&D as a collectible), and Garden Path slipped under the radar.
Also, many of the copies up for sale recently have been the same modules re-sold multiple times.
Foul