Dress of Bones calculates its final price assuming that these creatures can only be used once, which does not allow you to reload the ability of the tunic to bring creatures into battle. As such, its price is very low compared to a magic object that would allow you to "store for later"an existing creature.
I would use the Animal Feed spell as a comparison, not the dress of the bones. It allows you to "store" an animal for later teleportation in your country. The spell on which it is based, Contribution Object, has several small limitations, but increasing its spell level by 1 to remove some should be enough, for example, to be able to be used on an undead instead of one. animal, or have them actually stored in an interdimensional space while it is "stored" in the magical object, instead of being limited to a few tens of meters from you.
That would do that Bring the undead a 4th level spell, available at a minimum casting level of 7. For a magic item that would allow you to cast this movie at will (no charge or daily limitation), it would cost 50 400 gp (4 * 7 * 1800 gp). If you want this to be available only once a day, the cost would be 10,080 inches (divided by 5). If you want this to have 50 charges (like a wand), the cost would be 25,200 gp (divided by two). And so on, using the magic item pricing chart to adjust the price when the GM deems it necessary.
My question is: what would be the price to add a living dead to a dress?
This would be part of the spell activation. Once the ability is activated, the creature is stored in the dress and can be recalled later with the help of a Quick action.
Would this price increase if it became an article without a location?
The magic item would cost double whatever the price it should cost if it had a slot, as shown in the table relating to the price estimation of magic items.
How many undead or HD a dress should be allowed to wear to the maximum?
The spell itself has no limit, but each cast can only store one creature. This depends on the number of charges (total or per day) that you want the item to have.
Does the size of the undead affect the price or the number of a dress?
Yes, that should affect. The skeleton model does not change the creature's HD, but the zombie model does (because zombies are harder to kill). But here, the spell simply shops the creature did not create a new creature, the cost of creating the creature was managed by the spell or effect created.
Can we add to these undead equipment (such as weapons and armor) and would that affect the price?
Yes, it should. Adding powers or spells that require additional material should always be included in the final price. Since that would actually make the creatures strongerit should also cost more. But again, you do not create anything, you just store and recover an existing creature.
Do not forget that you must first work with your GM so that a new spell is created for this magic object, using the spell search rules if necessary. This spell would work in the same way as the summoner's summoning ability, which uses Dimension Door as a reference, also a 4th-level spell.
Otherwise, the only magical object that does something similar to what you described would be the Zombie Hide Shield (2,159 in.), Which does not allow you to "store" undead, but to reload its Unique zombie that he can summon by spending the corpse of a single dead humanoid to reload the shield.