Yes James, it is supply and demand.  "Demand" is not a measure of how many
people can afford one  after the price is set!  Demand is assessed BEFORE the
price is set!  It's a measure of how many people would want one, if they
COULD afford one.  That number, compared to the number of specimens actually
available, is (more or less) what determines the price.  If the number of
specimens available far exceeds the number of specimens wanted, the price
will be low, and everybody who wants one will get one.  In the case of
Cypraea leucodon and other rare species however, the number of specimens that
would be in demand at a lower price would far exceed the number of specimens
available - so the price is adjusted so that only the few people willing and
able to pay the high price will be able to get the few specimens that become
available.  The price has a way of finding its own level.  If the price goes
too high, even the few available shells won't sell.  If it's too low, all the
available specimens will sell out quickly, and people will be clamoring for
more.  So the system tends to balance the number of available specimens
against the number of buyers.  That's an oversimplification, but it really
does work out that way usually.  Hopefully folks who pay large sums for rare
shells are not doing so with the idea of investment in mind, because most of
the rare shells of yesterday are much more common now, and most of the rare
shells of today will not be as rare or as pricey tomorrow.
Paul M.