In general, slotless items cost more because they eliminate the opportunity costs that item slots usually represent: since you can only use one item per slot, and combining separate effects into one item both costs 50% more and also involves “putting more eggs in one basket,” it makes sense that a slotless item would cost more still, 100% more in this case.¹

As the magic item guidelines are just that, orientações, this will not always be true of every item that does not happen to fall into one of the traditional item slots. For example, something you need to hold in your hand in order to get the bonus cannot truly be called slotless, because it interferes with weapons and shields and all manner of other useful things characters do with their hands, and because it is conditional and may not happen to be where it needs to be when you need the bonus. If a weapon must be drawn to grant a Dexterity bonus, it won’t improve your initiative when you’re surprised—that is a grande downside. Such an item should cost menos, rather than more, than a traditional item in a traditional slot giving the same bonus.

But that does not describe your sword, since your sword grants its bonus even while sheathed. There is no hard limit on the number of sheathed weapons you can carry, and while the GM will presumably limit things if you really go overboard, having um extra sheathed weapon—this sword—while using whatever weapon you actually want to use (assuming it is not this sword) is presumably not going to run into that limit. As such, since you can get the bonus without it interfering in any other item you want to use, yes, this item is slotless, and the ×2 multiplier should apply.

Contudo,

as “an ancient artifact” that you “must attune” and “only one at a time” can be attuned by any given character, you are talking about an exceptional case for which there are really no guidelines and that cannot be handled so simply as assigning a gold-piece value to it. The game has a name for such items: artifacts. And the game does not provide pricing information for them (they are, literally, “priceless”).

The primary consideration for this sword, since you cannot use one of the other swords at the same time, is going to be what the other ones do, and which of them is best. Since they are based on the legacy weapons from Tomo da Batalha, I can tell you that Vento do deserto is definitely not the best of the weapons in that book—but more importantly, none of the weapons in that book are worth using, Porque Armas do legado was a very poorly-written book. Hopefully your GM is not following Tomo da Batalha too closely in their design, particularly the “personal costs.” But regardless of how the GM is handling it, the important question for this sword is going to be comparison to the others, not comparison to a pile of gold.

By the same token, concerns about wealth by level get a little blurry around artifacts. They are clearly inestimably valuable, but they necessarily are given out by the GM and unique. Opportunity cost is the name of the game. If the sword means you no longer have to buy a belt of incredible dexterity +2, then it’s worth at least 4,000 gp and the GM should think of it as such. If it allows you to buy a belt of mighty constitution +2 em vez de um belt of physical might +2, then it’s saved you 6,000 gp, and should probably be considered worth at least that much. If you have to buy a belt of incredible dexterity +4 or +6 anyway, then this feature is worthless and should be evaluated as such by the GM. If you never would have bought a belt of incredible dexterity, but now benefit from this bonus, it should be worth somewhere in between 0 and 4,000 gp. Unfortunately, figuring out exactly how much is going to be very specific to your character and the system offers little and less assistance in judging it.

On the other hand, the other reason you might need to know the item’s value is if you wanted to sell it. If you actually wanted to sell a unique artifact—whether this sword or something else—the real world has thoughts on how to handle pricing unique artifacts: namely, auctions. If that was your goal, as GM I would lay out a kind of quest for you to set up the auction, ensure everyone with the means and interest was aware of it, see to security, so on and so forth. D&D or Pathfinder isn’t really a great system for such a thing, but as a side-quest we’d make something work. And you would get a price based on bidding, which would be based on how well you hyped up the auction and got people willing to go and spend money. The Appraise skill would get a work-out for once!

1. _{Please note that in D&D 3.5e, Compêndio de Item Mágico actually waived the price premium on combining basic effects—like enhancements to Dexterity—with other items. Without the 50% premium on that, the 100% premium on slotless items of the same effect is much too high. How high it should be depends on how serious a risk item theft or destruction is in your campaign, but 50% seems like a clear maximum, and in most campaigns it should be quite a bit lower. Pathfinder has not re-implemented that rule, and in fact has gone strongly the other way by forcing ability score enhancements to only appear on two slots (belts for physical scores, headbands for mental scores), forcing the combining of magic effects—and the premium—far more than 3.5e ever did. This decision is indefensible; Paizo is wrong and Wizards of the Coast was right. For the sake of a better game, I strongly advise using the Compêndio de Item Mágico rule. For details on why the microfone rule is superior, you can see the explanation given in that book for it, or search on this site where the issue has been discussed at some length.}

The effect should cost +8,000 gp

A magic item effect that grants an aprimoramento bonus to an ability score, and does not require to be wielded, uses the following formula:

Ability bonus (enhancement): Bonus squared x 1,000 gp

No space limitation: Multiply entire cost by 2

An item that does not take up one of the spaces on a body costs double.

A +2 bonus, squared to 4, multiplied by 1,000 gp, and finally doubled: 8,000 gp

Of course, this is not counting for any other cost that this magic weapon should have, such as masterwork or other enhancement bonuses. A magic weapon requires a +1 enhancement bonus before you can add any other special property to it, so at least another 2,000 gp, plus the cost of it being masterwork (300 gp), and the cost of the mundane version of that weapon (15 gp for a longsword).

A sheathed weapon doesn't take any of the usual body slots, thus it's considered a slotless item. There are several other slotless items that have a weight and you have to pick it up and use it to get its effects (examples: Void Pennant, Faixa do Campeão, Ou o Apparatus of the Crab). The ioun stone there is merely an example.

Slotless Wondrous Items

Wondrous items not worn in one of the magic item slots are called “slotless” wondrous items. Sometimes these items take the form of trinkets, like figurines of wondrous power. Other times they are larger items, such as the carpet of flying. Typically the possession of such an item is enough to gain its benefit, but sometimes one must manipulate and activate the item.

Slotless wondrous items are the largest category and most diverse of the wondrous items.

Otherwise, if the weapon must be wielded to grant its effects to the wielder, it should be treated as an item that takes up a slot (a weapon slot). This can be seen if the price of the Língua da Chama (Equipamento final, pg.155) is reverse engineered:

+1 flaming burst longsword

masterwork longsword 315gp, +3 magic weapon 18,000gp

2 (spell) * 3 (CL) * 2000, divided by 5, 2400, plus 18315, total of 20715 gp.