Two entries in an old catalogue have different prices. Before deciding which product cost more, find out what each price buys. A figure beside a drawing can refer to one object, a pair, a dozen, or an assortment described elsewhere on the page.
Read beyond the price column
Inspect the heading, nearby abbreviations, and any note governing the section. Keep the currency and selling unit in the same transcription as the number. If the unit is unclear, leave the comparison unfinished rather than assume a single item.
For a deliberately invented arithmetic example, one entry offers twelve buttons for 24 units of currency; another offers one button for 3. The first works out at 2 per button if the package contains twelve equivalent pieces. That calculation describes the example, not a historical market.
Check whether the products are comparable
Quantity is only one condition. Different sizes, materials, finishes, or packaging may explain differences even after you calculate a per-item figure. A discount for a larger order also does not establish what a customer actually bought.
Our discussion of catalogue illustrations showing a product family is useful here. The drawing may represent several listed variants, so connect the price to the exact written entry rather than the nearest picture alone.
Keep historical interpretation separate
A catalogue records an offered price under particular terms. It is not automatically a receipt, a measure of affordability, or a present-day valuation. Those questions require additional evidence and a stated method.
A careful caption might say that the listed amount applies to a dozen of one specified size. That modest sentence is more informative than calling an object cheap or expensive without knowing the unit in which it was sold.