saved

What this one hit did not cost, in currency, or 0.0 when the scope declares no TokenPrices.

Per hit rather than cumulative, because that is what an event is: a subscriber that wants the running total reads CacheStats.savings, and one that wants a counter to add to adds this. It is the cost of the specific call that was avoided, computed from the token counts on the entry that was served, so a hit on a long answer is worth more than a hit on a short one.