Adding to your collection
Looking at the catalogue
Catalog value is the price that collectors can expect to pay for
a stamp. Catalog value can be highly variable, going up and down
with the marketplace.
In StampData (at present), catalog value is recorded as a very broad approximation, basically the "number of zeroes in the price", assuming a USD/EUR as base. So "e3" denotes a price in the 1-10$/E, while "e5" is a several-hundred-dollar stamp. It is also possible to record differing prices for mint and used, so "e*4o3" is is the tens range if mint, but 1/10 of that if used.
This has the advantage of stability over longer periods of time, lets the collector see where the high-priced stamps are, and avoids the false precision of a normal price tag.