How do you price magic items with continuous effects based on spells with duration: concentration?

Question

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 custom magic item pricing guidelines do not specify a cost modifier for effects with duration: concentration. Now, in practice, concentration is usually a more restrictive duration than rounds/level. How should we account for this?

The guidelines are, of course, guidelines, and the best answer may go beyond them. However, I am particularly interested in precedents from the rules as written, including book items.

For instance, one relevant example is the Hand of the Mage. The guidelines indicate that a continuous item of mage hand should cost 2,000 gp (continuous item) * 0.5 (spell level) * 1 (caster level) = 1,000 gp. However, the item is priced at 900 gp and instead allows you to cast the spell at will (which is uncommon, and for which there are no guidelines). Perhaps this is a good solution for spells like mage hand that don't do anything by themselves, but grant you an ability that you can use at-will while concentrating.

Answer 1

As pointed out by Mike Kellogg in a comment

Hand of the mage is a command word item, which means 900 gp actually is the correct price by the formula for command word items (½ × 1 × 1,800 gp).

^{(minor formatting by me)}

Anyway, if creating an actual continuous item of a concentration-requiring spell, that would imply that its effect doesn’t require concentration. If it did, it would be at-will, not continuous (and thus the footnote doesn’t apply).

For many concentration-requiring spells, I suspect that being truly continuous will be kind of nonsensical. But assuming you have one that isn’t (or you have an understanding of how it will work without requiring concentration), I would call that a huge advantage, arguably even greater than making a rounds/level spell continuous (though at low caster levels, not much greater). As such, the ×4 factor would be where I would start, but I’d consider going up from there.