Hi Nora,
The only Pleurotomaria that is commonly offered in decent condition for under
$100.00 is Pleurotomaria westralis from western Australia.  It is probably
more common because it lives on flat bottoms where it can be dredged or
trawled, instead of the steep rocky inclines and ledges where many of them
live.  The next commonest is probably P. hirasei, which can often be obtained
for under $200.00.
Regards,
Paul M.
Rhode Island