According to Adding New Abilities, to add to an existing magic item the effects of a polymorphic pouch—an already slotless item—costs only 5,000 gp—, that is, the price of the of polymorphic pouch.
Precisely what such an effect does when applied to an existing magic bow is up to the GM, but this GM would allow a +1 polymorphic bow to remain unmelded when the owner changes forms and allow the bow's wielder to take a free purely mental action to retrieve one of the 100 arrows that's stored in the bow's extradimensional space. (This saves the buyer from having to buy a second polymorphic pouch effect for his quiver!) In addition, because the GM's essentially more than halved the extradimensional space's capacity (the GM thinking 100 arrows more elegant than the 266 arrows a pouch could normally contain) and limited the extradimensional space to only arrows but allowed the wielder to retrieve an arrow stored in the space as a free action rather than as move action that provokes an attack of opportunity, the GM would likely reduce the price of adding the pouch's effect to a magic bow by 10% to 4,500 gp, making that function, for example, less versatile but with more raw arrow storage than the 1,800 gp efficient quiver.
"But how do I determine if a magic weapon special ability should be an enhancement-bonus equivalent (i.e. a magic plus) or a flat cost?"
If you're a player, you pitch the original item to the GM, and the GM will set the price. If you're the GM, you set the price by comparing the proposed original item to existing items.
More specifically, so far as I'm aware, there's no magical-enhancement-bonus-equivalent-to-flat-gp-value conversion in Pathfinder, but its antecedent D&D 3.5e provides a very basic guideline over the course of the 1st-level infusion personal weapon augmentation [trans] (Eberron Campaign Setting 117), the 4th-level infusion weapon augmentation [trans] (ibid.), and the 6th-level infusion greater weapon augmentation [trans] (ibid.). The first infusion allows an artificer to add to a weapon a weapon special ability "whose market price is equivalent to a +1 bonus or up to 10,000 gp," the second up to +3 or up to 70,000 gp, and the third up to +5 or up to 200,000 gp. (In either game, the referee must still adjudicate the gp equivalents to +2s and +4s!)
(By the way, armor and shields are similar except they're 1st-level, 3rd-level, and 5th-level infusions (e.g. lesser armor enhancement [trans] (Eberron Campaign Setting 109) et al.) and the flat gp value is half that of the equivalent magic weapon.)
Using the weapon augmention infusions as guidelines, the Pathfinder GM may rule that a polymorphic pouch effect applied to a magic bow is, instead, a weapon special ability worth a +1 enhancement bonus.