Tom,
Aren't you glad you didn't buy that gloriamaris!!
I paid $75.00 apiece for my first two Cypraea teulerei, 30 years ago.
 Now you can get a nice one for $10.00.  You are right about shells
not being a good investment (financially speaking).  They almost
always decrease in value over time.  A few species may be on the
brink of extinction, particularly terrestrial species; but usually if
a marine species is still living, then somewhere there has to be a
population dense enough to ensure that individuals can find mates.
The "loners" that are collected over a widespread area usually just
represent individuals that were carried away from the breeding area
as floating larvae.  Conus gloriamaris was being collected at the
rate of one or two specimens per year, until a couple of guys went to
a small unpopulated island in the Philippines and collected 80 of them
in one afternoon, and the price dropped accordingly.  Today, many
areas are known where that species can be collected, and the shell,
while still uncommon, is not a great rarity.  There may be a locality
where Conus cervus is equally common.  If so, eventually someone will
find it, and the price will drop.  As you noted, some values do
increase over time, but usually not as fast as inflation, therefore
the net effect is a decrease in value.
There is an old story, I'm not sure how true, about Epitonium
scalare.  As the story goes, some European emperor owned the only
known specimen.  When a second specimen became available, the emperor
paid a fabulous price at auction for the shell, then put it on the
floor and stomped on it, thereby ensuring that his first specimen
remained the one and only.  Hopefully most of us approach our
collections with a bit more reserve than that.
Paul M.
Providence, Rhode Island, USA