The CRB says 6g per unit on top of the cost of the ammo.
Does that mean "per arrow" or "per bundle?" A quiver of arrows can technically be a unit. Would it be 121g (20 arrows x6g, +1g for the cost of the bundle) or would it be 7g for the bundle? One seems too high, the other too low.
I ask because I personally can't see someone charging 6g, 1c for a masterwork sling bullet (no matter how fancy) when you get 10 normal ones for a silver piece.
Masterwork cost and special material cost are per each arrow or bolt.
50 arrows* x 6 gp for masterwork = 300gp (not including the arrows themselves.
Masterwork longsword = 300gp (not including the longsword itself)
*Ammunition is enchanted in batches of 50. So 50 +1 arrows is 2300gp, plus arrow cost. A +1 longsword is 2300gp, plus sword cost.