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.