The price guidelines for magic items with use-activated or continuous spell effects are as follows:
Spell level × caster level × 2,000 gp 2
2 — If a continuous item has an effect based on a spell with a duration measured in rounds, multiply the cost by 4. If the duration of the spell is 1 minute/level, multiply the cost by 2, and if the duration is 10 minutes/level, multiply the cost by 1.5. If the spell has a 24-hour duration or greater, divide the cost in half.
The example item given is the Lantern of Revealing. It's one of the simplest items - duplicating the 3rd level spell Invisibility Purge from a 5th level caster exactly when it's in use. Since Invisibility Purge's duration is 1 min./level, its cost should be calculated as such:
Spell level 3 × CL 5 × 2,000gp × 2 = 60,000gp
Seems easy enough.
Except it's not. It's listed as 30,000 gp.
Am I missing something obvious or some errata somewhere? Do other use-activated/continuous magic items follow the given formula? It seems odd that the extraordinarily textbook example item isn't by the book, as it were.