Paul is certainly right about the inevitable decrease in (cash) value
of shells over time.  This doesn't matter to me, since cash value has
nothing to do with the value I place on specimens I own.  Like any
commodity, shells must have an assigned monetary value in order to
exchange hands, and as in other commodities, that value reflects
current supply and demand.  As Paul indicated, the supply of a given
shell will almost certainly increase over time, so if the demand
remains fairly constant, the cash value will thereby decrease.  If
you want to collect something as a financial investment, shells are
therefore a singularly poor choice.  And, it is precisely because of
that fact that most shell collectors (in contrast to collectors of
many other kinds of objects) are not in the hobby because of the
monetary angle.  A price on a shell is a necessary evil, something we
have to grin and bear, a means of accessing the true value.  But the
true value of a $300.00 golden cowrie, or of a $1.00 dawn cowrie,
lies in the shell itself, in how it looks, how it feels in the palm
of my hand, how it affects my psyche and my imagination when I hold
it - not in the dollars I paid to obtain it.  The two species just
mentioned are two of my personal favorites, which is to say both of
them are highly valued by me.  It is like purchasing a comfortable
chair.  You have to part with dollars to get it, but once you have
it, the momentary, objective MEANS of obtaining it becomes
irrelevant, in light of the enduring, subjective REASON you obtained
it.  Those who collect objects only as a financial investment cannot
appreciate this aspect of collecting.  The real value of my own
specimens will always remain the same, regardless of how many dollars
other collectors are exchanging for similar specimens.  If I collected
as an investment, the value of my specimens would depend entirely on
the current market exchange rate.  That is such a depressing thought,
I don't even want to dwell on it.
As for collecting stamps with pictures of shells on them, I do not
have a real appreciation of stamp collecting, nor an understanding of
the subjective aspects of that pursuit, so I cannot be objectively
critical.  However, subjectively speaking, as an admitted philatelic
ignoramus, collecting mere manmade pictures of something, whether it
be on postage stamps, coffee mugs, jewelry, or whatever, has little
in common with collecting the actual entity.  A postage stamp, no
matter how rare, is TO ME a mere scrap of manmade paper on which a
manmade machine has impressed some manmade colored ink.  Comparing
that to the intrinsic wonder of a natural object created in the
depths of the ocean by incredibly complex processes that have been in
operation for millions of years, and which humans are only beginning
to comprehend, somehow seems bordering on sacrilege.  What
relationship is there between a Renoir hanging in the Louvre, and a
piece of sticky colored paper, created as a receipt of payment for
services, depicting that same work of art?  Please! - I am NOT
bashing stamp collectors!  They obviously see something in their
objects of interest that I am incapable of seeing.  These are only my
own subjective thoughts, and pertain only to ME!
Regards,
Paul Monfils