James,
 
Drupa used to be in the family Thaididae (still are in my collection but
I'll get it straightened out in 20 or so years) but are now placed in the
subfamily Rapaninae (still Thaidinae to some) under the family Muricidae.
According to Emily H. Volkes  ("One Last Look at the Muricidae" in an
article on the COA web page) there are 5 species of Drupa, 4 of Ricinella,
and 2 of Drupina.  So if you are just talking the genus Drupa, I don't think
any are very rare.  I couldn't find the Volkes article on the COA page, it
may have been taken off (glad I made a hard copy) but I recommend you check
out the site.  This link will take you to the Conchology 101 section of COA
by Dr. Gary Rosenberg.  This is perhaps the most concise, easy to understand
overview of this subject you will ever find.  As for the rest, use the web.
If you had run a search for drupa you would surely have found Hardy's site
with all of his pictures and he would reference Poppe's site.
 
Look for images at:
 
http://www.club.innet.be/~year0078/
 
http://www.gastropods.com/shell_pages/Taxon_pages/Family_MURICIDAE_RAPANINAE.html
 
The first site is Guido Poppe's page where you need to go to look for any
shell images, he has thousnads and thousands of quality images.  The second
site is Eddie Hardy's site and will link right to the Drupa list.  He only
list gastropods but has thousands of images.  Both sites will give you the
taxonomic status of a shell, a species list (easiest to use on Poppe's
site), and references (easiest to use on Hardy's site).
 
As for rare, I don't think so but there are a couple of Drupa that are
uncommon and not seen very often.
 
Tom Eichhorst in New Mexico, USA