Adding spells to items

[Mr. woop woop]

the wording for adding a spell to an item is very clear. (Use-activated or continuous = Spell level¹ × caster level × 2,000 gp³)
The wording for Use-activated is also pretty clear.

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This type of item simply has to be used in order to activate it. A character has to drink a potion, swing a sword, interpose a shield to deflect a blow in combat, look through a lens, sprinkle dust, wear a ring, or don a hat. Use activation is generally straightforward and self-explanatory.

Many use-activated items are objects that a character wears. Continually functioning items are practically always items that one wears. A few must simply be in the character’s possession (on his person). However, some items made for wearing must still be activated. Although this activation sometimes requires a command word, usually it means mentally willing the activation to happen. The description of an item states whether a command word is needed in such a case.

Unless stated otherwise, activating a use-activated magic item is either a standard action or not an action at all and does not provoke attacks of opportunity, unless the use involves performing an action that provokes an attack of opportunity in itself. If the use of the item takes time before a magical effect occurs, then use activation is a standard action. If the item’s activation is subsumed in its use and takes no extra time use activation is not an action at all.

Use activation doesn’t mean that if you use an item, you automatically know what it can do. You must know (or at least guess) what the item can do and then use the item in order to activate it, unless the benefit of the item comes automatically, such from drinking a potion or swinging a sword.

So could I make an Use-activated bow that casts arrowsplit when used to attack.

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Arrowsplit
Conjuration (Creation)
Level: Assassin 3, ranger 3, justice of weald and woe 3
Components: V, M
Casting Time: 1 swift action
Range: Touch
Target: One masterwork arrow or bolt
Duration: Instantaneous
Saving Throw: None
Spell Resistance: Yes
This spell is cast upon a masterwork arrow or bolt, causing it
to split in mid-flight into 1d4+1 identical masterwork arrows
or bolts. All the missiles strike the same target, and you must
make a separate attack roll for each missile. The arrow or bolt
must be fired during the same round the spell is cast, or the
magic dissipates and is lost. The projectile is destroyed even if it
misses its target.
Material Component: Masterwork arrow or bolt.

if so doesn't this multiply all bow damage by 2-5x for only 66k?(If you're not crafting it.)

[Leviathan]

This question occurs more often as "Can I make a use-activated item of true strike?". The answer is that the magic item creation guidelines are just that: guidelines.

[T]hese few formulas aren’t enough to truly gauge the exact differences between items. The price of a magic item may be modified based on its actual worth. The formulas only provide a starting point.

I would think that use-activated arrowsplit is actually worth a lot more than 66,000 gp. If you're a player, you'll need to work with your DM to figure out exactly how much such an item should cost (or if it's even available at all). If you're the DM, you'll need to make that decision yourself.