Man-made objects attached to carrier shells influence the sale price to the
extent that a carrier shell collector is willing to pay.  Seriously, this is
entirely subjective.  If a dealer has a carrier shell with unusual
attachments, he may list it at a higher price, hoping someone will be
interested enough to pay more for an unusual specimen of a common species.
More likely, the dealer will simply call a known Xenophora enthusiast and
offer the specimen directly.  The average collector, who simply wants the
species represented by one specimen in his/her collection, would often
prefer a "typical" specimen, and would want to avoid specimens with atypical
attachments.  So it's really a matter of personal preference.  Specialists
are often willing to pay considerably more than a specimen is technically
"worth", if the specimen is something unusual in their particular area of
specialty.
As for rare shells attached to carriers, most carrier attachments are not in
very good condition.  I had a Xenophora pallidula with three small Conus
bengalensis attached.  But, the cones, if removed from the carrier, would
have been considered "junk", and no cone collector would have looked twice
at them.  Their value therefore, separated from the carrier, would have been
zero.  Their only value was in making the carrier shell unique (as every
carrier shell is) and interesting.  I sold the shell for $100.00 to a
Xenophora specialist, but the price was not so much a matter of the
attachments themselves being rare specimens, as the carrier being a rare
specimen by virtue of the unusual attachments, if that makes any sense.
There is no objective way to determine the value of such a specimen.
Xenophora pallidula typically lists for about $15.00.  If I had offered this
particular specimen at $200.00, this specialist still might have bought it.
So what was the "true" value??  Now, if you found a $15.00 carrier shell
with an attached GEM specimen of a $200.00 shell, the value of the
attachment would surely outweigh the value of the carrier.  But that doesn't
happen very often.

Paul M.