I would start by checking the Beaststrike Club, which is a club that has several wild-shaping themed effects. It can transform into the head or limb of an animal, and a druid with wild shape can increase the weapon size by one category.
It costs 7,300 gp. Which means the special ability costs 5,000 gp (300 gp masterwork, 2,000 gp for +1 enhancement). This is also similar to the cost of a Resizing weapon (+4,000 gp).
If we check the Wild special ability, we will notice that Wild costs about 15,000 gp. As a Wild armor should at least be priced as a +4 armor (16,000 gp) and 1,000 gp is the cost of a +1 bonus.
Now, we know that weapon abilities cost at least twice as much as armor abilities, even though armors usually keep your character alive so he can keep killing his enemies. That said, a +3 weapon ability would cost way too much (about 30,000 gp), for the simple effect of being able to be used.
We would have to ask the developers to know why Wild was priced so high, but we can guess that it's because armors are gone when wild shaping. At the earlier levels, all you gain to compensate for your now missing armor are small natural armor or dex and size bonuses, +2 or +3 to AC. While at later levels this bonus increases to +5 or +7. A druid's answer for this lack for armor AC is the spell Barkskin, but they have no other option for missing special abilities other than Wild.
Meanwhile, your manufactured weapons are replaced by new natural weapons, so you are losing little if the weapon doesn't have magical special abilities, and sometimes you will even get stronger weapons. Not only that, but druids have the option to obtain properties like Flaming or Keen by using an Amulet of Mighty Fists, which is twice the price of a weapon with the same properties, but will apply the effects to all natural attacks, and they also have access to Magic Fang. So, weapons are pretty much covered in terms of damage for wild shaping druids.
I wanted to say all this simply to point out that there is no need to price the weapon special ability as high as the armor special ability. Because there are other items that already cover the weapon balance for wild shaping druids.
With all that said, let's brainstorm a Wild weapon special ability.
First, what do we want this ability to do? Not transform, right? As in, not meld into the caster's body and actually be available to be wielded if the new form is capable of wielding weapons.
You are not gaining much by wild shaping with a greatsword (2d6) as you will not be able to wield it a the same time as you can make use of your natural attacks effectively, which usually cause 1d6 damage, but allows multiple rolls since you have multiple weapons. If you wild shape a dagger (1d4), the potential damage output is reduced. Which means the usefulness of the special ability will depend on which weapon it is enchanted on.
Of course, why would we enchant a weapon with Wild if not to use other special abilities with it? That's right, that is the whole point of it.
A Wild weapon is, at minimum (assuming a +1 equivalent bonus), a +2 weapon and will cost 4,000 gp (plus masterwork), and any additional special ability applied on it will increase it's cost significantly. A +1 flaming magic sword costs 4,000 gp. A +1 flaming wild magic sword will cost 9,000 gp, a 5,000 gp difference. See the similarity in price with Resizing and Beaststrike?
This difference only goes up as the enhancement bonuses increases:
- +3: 7,000 difference (9,000 vs 16,000)
- +4: 9,000 difference (16,000 vs 25,000)
- +5: 11,000 difference (25,000 vs 36,000)
- +6: 12,000 difference (36,000 vs 48,000)
- +7: 16,000 difference (48,000 vs 64,000)
- +8: 17,000 difference (64,000 vs 81,000)
- +9: 19,000 difference (81,000 vs 100,000)
- +10: 21,000 difference (100,000 vs 121,000)
Depending on your build, it might be too expensive for what it does. But as the enhancement bonuses increases, you are allowed to wield an even more powerful weapon than you normally could with an Amulet of Mighty Fists, especially after +6, as the cost is over 10,000 gp, but that is already past the limit of the amulet (maximum +5) and makes the ability stronger than the amulet.
To me, that is worth a +1 bonus.