I think Oli came very close. The EGU prefix was once thought very scarce in full sheets until an eBay dealer started offering large quantities of them. One particular lot consisting of 25 sheets sold for $7,168, or less than $290 (US) each. When large quantities suddenly appear of a commodity for which there is weak demand, the price will not hold.
So why did the price only fall to $400, and not $290? Twenty-five sheets is a wholesale lot, and they could be expected, given time and promotion, to retail individually for a somewhat higher price.