savings

What the hits in each scope did not cost, for every scope the caller declared TokenPrices for.

Empty unless prices are declared, and it stays empty rather than guessing: the library ships no table of provider prices, because one would be wrong the month after it shipped and a cache that reports the wrong saving is worse than one that reports none.

Keyed by scope, and never summed across scopes here. Two scopes may declare different currencies and adding those together would be arithmetic on units that do not share a meaning. Each Savings carries the prices it was computed from, so no total can be read without its assumption.