Some magic items, such as the oil of sharpness, can be found with a variety of doses, and the treasure value list explicitly states the value (in XP and GP) of each dose. In other cases, the DM will need to do some math. The value listed in these cases is always for the maximum number of doses.
In the cases of items such as magic arrows and sling bullets, the listed price is for a single missile.
Certain items, such as the ring of protection, come in varying forms and power levels. In the case of the ring of protection, the value (in XP and GP) for the varying forms is listed. In other cases, such as with the ring of wizardry, the DM will need to do some math. The value listed is always for the most powerful of the items.
Sometimes the math is not so obvious, such as in the case of the ring of wizardry. How much more valuable is one of the better rings over the weaker ones? We have used 15,000 GP as the value of one of the weaker rings, as an example. Keep in mind that a ring granting 2nd level spells is not strictly better than a ring granting 1st level spells – a certain 1st level spell may be much more useful in a scenario than any available 2nd level spell!