There's a note in the item creation guidelines to the effect that the formulas are a starting point, not the end-all, be-all of balance.
For example, a command-word item of Cure Minor Wounds (caster level 1, spell level 0.5, command word - 1*0.5*1800=900 gp) is within the price range of even a 1st level character... but it breaks the balance of combat classes (infinite healing, free of continuing charge, outside combat - a minute, and a 1st level character is fully healed. Ten, and a 10th level character is healed. Half an hour, and a 20th level character is fully healed.) Such an item is more useful than the guidelines suggest.
A Ring of Invisibility is listed at 20,000 gp. It's caster level 3, and a 2nd level spell. Command-word, that would be estimated at 2*3*1800=10,800 gp. Continuous, that would be listed as 2*3*2000*2=24,000. It's priced somewhere in between... but it has to be activated, and it's "as the spell" so theoretically it's a command-word item. It does not follow either set of guidelines. Invisibility at will is more useful than the guidelines suggest.
Bags of Holding, the Quiver of Elhonia, Heward's handy Haversack, and Portable holes aren't priced by guidelines. They're priced by expected usefulness.
