As an item, this is going to end up less powerful than it appears at first blush, and 24,000 gp is too expensive for what it gives you.
Let's unpack what this spell does and doesn't do (emphasis mine):
You create a melee weapon sized appropriately for you from opaque force.
You are considered proficient with this weapon, which acts in all ways as a masterwork weapon typical of its type. The instant weapon has hardness 20 and the same number of hit points as a typical weapon of its type. As a force effect, it can strike and damage incorporeal creatures. If the instant weapon leaves your hand at any time, the spell ends at the beginning of your next turn.
While the spell is in effect, you gain an ordinary masterwork melee weapon sized to fit you which you can use as if you were proficient. It doesn't actually grant you any feats needed to use it, nor are you considered to have them for any other purposes, such as using a katana one-handed or using a sawtooth saber as a light weapon. You still need the Two-Weapon Fighting feat(s) to get the most out of a double weapon or Improved Trip to make a trip weapon worthwhile.
It is also a steel or wooden weapon with no enhancements, so you will be unable to bypass any damage reduction that can't be overcome by mundane weapons (such as DR /bludgeoning). Since the weapon disappears when you set it down, you will need the actions of another player to applying blanching or cast align weapon or magic weapon each time you face a foe with damage reduction.
This also leaves you highly susceptible to disarm attacks, since it costs you a standard action to get your weapon back rather than a move action to pick it up. A decently skilled foe could ensure that you are unable to attack for the duration of the battle.
All this item gives you is versatility in your reach and the type of mundane damage you deal. A +1 ghost touch weapon would cost just over 8,300 gp and allow always-on bypass of DR /magic. There are no exotic weapons that are really abusive without the feats and materials to really crank up their power. You're much better off just getting a weapon to focus on.
Pricing Breakdown
All that said, I would price this item closer to 8,000 gp just for the proficiency with the conjured weapon. Compare this item to the bladed belt, which "can be enchanted like a piercing and slashing melee weapon" and only costs 2,000 gp. Your item provides weapons that can't be enchanted but lets you use them as if you were proficient. In fact, 24,000 gp could get you a mattock of the titans, which is substantially more powerful than your proposed item.
What you need to price is what the item actually does:
- conjure a nonmagical weapon
- provide proficiency with that weapon
#2 is easiest to price, so let's start with that. The nearest item to that effect is the lesser bracers or archery. At 5,000 gp, they grant proficiency with all bows and crossbows and a +1 competence bonus on attacks if you're already proficient. That prices out to roughly 1,600 for the attack bonus (bonus squared x 2,000 gp, minus a 20% reduction for requiring proficiency) and 3,400 gp for the proficiency. Since it's far broader than what your item gives you, I'd drop that to an even 3,000 gp.
#1 is trickier. There are no magic items that I could find which even partially bring a weapon into existence. A crown of swords gives you retributive spiritual weapons, but only 10 rounds a day, for 6,000 gp. Your item isn't that good, but it does let you use it all day (with repeated activations). I'd suggest a 5,000 gp price tag for that. It's useful but not powerful and isn't open to abuse as far as I can see.
That brings your total cost to 8,000 gp, which is slightly less than the +1 ghost touch weapon.
