Charles,
It's the difference between "number of specimens surviving" on the one hand, and "desirability to collectors" on the other.
Rarity ratings in the new
RIC are based on "number of specimens survivng and known to the authors". Your
Titus sestertius is indeed very
rare, as confirmed by the die identity of your specimen with the
Paris coin illustrated in
RIC, and by the absence of that
type in the BM
collection up to 1976 (p. 140 in
BMC).
Now the
rarity ratings in the old
RIC should theoretically have been based on the same criterion, but in fact they were heavily influenced by Cohen's franc prices. Market prices of course do not reflect absolute
rarity: they are much more dependent on the historical or artistic interest of the
type, determining how eager collectors will be to add it to their
collections.
Titus' standing
Roma type on
COS II sestertii is very
rare, but not interesting: so
Cohen 181 correctly gave it the
low price of 4 francs, and the old
RIC unfortunately followed suit with rating "common"!
Many varieties of
IVDAEA CAPTA types on
sestertii of
Vespasian and
Titus are quite common. They are of great interest to collectors, however, so
Cohen priced them at 8, 12, or 20 francs. The old
RIC again unfortunately followed suit, rating these
types "
scarce" or "
rare"!
