
Mauril |

It's not entirely clear, but it is suspected that the alchemical silver price has an accidental omission of "per missile" like adamantine does. This is because, in 3.5, both adamantine and alchemical silver list their prices as +60 gp and +2 gp for ammunition. Paizo changed the chart on adamantine to make it more clear and, it is suspected, they neglected to do so for alchemical silver.
The other thread I could find on the subject had no official word, so it is going to ultimately be left to DM interpretation.