Nora,
 
I also started with an idea of collecting a representative sample of each family
and I think this is still a worthy goal.  Do not despair over getting some of
the expensive ones like the Pleurotomaria.  I presently have three Pleurotomaria
in my case and none cost over $40.  The first was a P. westralis that was only
$40 back in the early 1990s when I bought it.  They are now much higher but
things change - note the Cypraea fultoni.  My next one was a P. africana with a
dinged and repaired spire - it looks great and was also $40.  My last was a P.
teramachi with a hole busted in the base ($33) again it looks great and I have
another on the way with a busted lip.  These shells came from dealers currently
selling shells.  They like to deal with F+ or gem specimens but they get others
in not so fine shape.  Just bide your time or if you deal with a particular
dealer let him or her know what you are looking for.  I hate to do that as I
bargain shop all over the place with several dealers and I don't feel quite
right leaving any of them with a private wish list.  I know any of them would be
glad to do this but it is just something I haven't felt quite right about. All
are super people and if I haven't bought something from one in a while I get to
feeling very guilty and try to come up with some kind of order.  I seldom spend
much on my shell orders but every dealer has always treated me like they expect
me to drop a bundle next time -- in other words very nice.  So check out a few
of these guys, I have a long list of most of them I can send you or you will
find them on Tom Rice's page or Bret Raines Mollusc Net.  If you want my list,
let me know.
 
By the way, you will find there are other families much tougher to collect than
Pleurotomaria.  Not because they are so expensive, but because they are obscure,
small, and not generally attractive.  Probably the toughest for me was when I
decided to get a representative Caecum shell.  It is a bit disheartening to pay
$2 for a shell I can barely see with my naked eyes.  This one at least came with
an enlarged photograph.  I won't be adding too many more of those real soon.
The entire micro shell area of collecting is a tough one and really demands a
decent microscope.
 
So as of now, I have a rep for most families (and a bunch from some) but I am
still missing a few obscure ones, you will also find some bivalve families tough
to find - at least I have.  I keep looking and eventually all will fall in
place.  So hang in there,
 
Tom Eichhorst in New Mexico, USA