Tom's comments about rare nerites illustrate that shells tend to be priced
according to the same age-old criteria that sets prices for most other
commodities - supply and demand.  Low supply and high demand tend to boost
prices, but really high values result only when BOTH these criteria are
present simultaneously.  The mere fact of rarity (low supply) will not
result in a high monetary value unless there is also a high level of demand
for the same shell.  That's why nerites, even relatively rare ones, are
generally low priced.  They are not a very popular family among collectors.
In addition, many of them can be found literally by the thousands in readily
accessible intertidal areas (literally littoraly one might say).  From a
strict supply and demand viewpoint, such a shell is probably worth about
$.05.  But as Tom pointed out, a dealer isn't going to go to the trouble of
importing, cleaning, listing, labeling, and shipping a shell for that price,
or even for ten times that price.  So, dealers who bother with such shells
at all generally have a minimum price of $1.00 or so, and some will offer
lots of two or three specimens for $1.00.  A cowrie shell which is just as
common as a $1.00 nerite may list for two or three dollars.  Why?  Because
cowries are a very popular family (higher demand).  With cones and Murex
shells, prices tend to be somewhat higher for shells of equal rarity,
because these shells are more subject to damage (thin lips of cones; spines
and encrustations of Murex).  So, even though the species may be equally
common, the gem quality specimens which many collectors prefer are not
equally common.  Very high demand for a species can also push prices up,
even for shells which are not particularly rare, like Cypraea aurantium
(golden cowrie) and Conus gloriamaris (glory of the sea cone).  A large gem
golden cowrie might list for $300.00.  An equally rare and equally gem
Cymatium might sell for $50.00, while a rare gem nerite might bring $10.00.
On the other hand, a collector of modest means might consider a somewhat
flawed Cypraea aurantium a real steal at $75.00, while an equally flawed
common shell might not interest anyone (why buy a flawed one when you can
get a perfect one for $5.00?).
Paul M.